fix clippy warnings

[jjl]

for example, with the connected nodes example:

warning: this `if` statement can be collapsed
  --> src/main.rs:11:4
   |
11 |    path(x, y) <-- edge(x, y);
   |    ^^^^^^^^^ help: collapse nested if block: `if path(x, y && path(x, y && path(x, y path(x, y`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#collapsible_if
   = note: `#[warn(clippy::collapsible_if)]` on by default

warning: this `if` statement can be collapsed
  --> src/main.rs:12:4
   |
12 |    path(x, z) <-- edge(x, y), path(y, z);
   |    ^^^^^^^^^ help: collapse nested if block: `if path(x, z && path(x, z && path(x, z path(x, z`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#collapsible_if

warning: using `clone` on type `i32` which implements the `Copy` trait
  --> src/main.rs:11:4
   |
11 |    path(x, y) <-- edge(x, y);
   |    ^^^^^^^^^
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#clone_on_copy
   = note: `#[warn(clippy::clone_on_copy)]` on by default
help: try removing the `clone` call
   |
11 ~    ascent! {
12 +    relation edge(i32, i32);
13 +    relation path(i32, i32);
14 +       
15 +    path(x, y) <-- edge(x, y);
16 +    path(x, z) <-- edge(x, y), path(y, z);
17 ~ }) <-- edge(x, y);
   |

warning: using `clone` on type `i32` which implements the `Copy` trait
  --> src/main.rs:12:36
   |
12 |    path(x, z) <-- edge(x, y), path(y, z);
   |                                    ^ help: try dereferencing it: `*y`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#clone_on_copy

warning: using `clone` on type `i32` which implements the `Copy` trait
  --> src/main.rs:12:4
   |
12 |    path(x, z) <-- edge(x, y), path(y, z);
   |    ^^^^^^^^^
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#clone_on_copy
help: try removing the `clone` call
   |
12 ~    ascent! {
13 +    relation edge(i32, i32);
14 +    relation path(i32, i32);
15 +       
16 +    path(x, y) <-- edge(x, y);
17 +    path(x, z) <-- edge(x, y), path(y, z);
18 ~ }) <-- edge(x, y), path(y, z);
   |

warning: using `clone` on type `i32` which implements the `Copy` trait
  --> src/main.rs:12:27
   |
12 |    path(x, z) <-- edge(x, y), path(y, z);
   |                           ^ help: try dereferencing it: `*y`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#clone_on_copy

warning: you seem to use `.enumerate()` and immediately discard the index
 --> src/main.rs:8:13
  |
8 |    relation edge(i32, i32);
  |             ^^^^
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unused_enumerate_index
  = note: `#[warn(clippy::unused_enumerate_index)]` on by default
  = note: this warning originates in the macro `ascent` (in Nightly builds, run with -Z macro-backtrace for more info)

warning: using `clone` on type `i32` which implements the `Copy` trait
 --> src/main.rs:8:13
  |
8 |    relation edge(i32, i32);
  |             ^^^^
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#clone_on_copy
help: try removing the `clone` call
  |
8 ~    relation ascent! {
9 +    relation edge(i32, i32);
10+    relation path(i32, i32);
11+       
12+    path(x, y) <-- edge(x, y);
13+    path(x, z) <-- edge(x, y), path(y, z);
14~ }(i32, i32);
  |

warning: you seem to use `.enumerate()` and immediately discard the index
 --> src/main.rs:9:13
  |
9 |    relation path(i32, i32);
  |             ^^^^
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unused_enumerate_index
  = note: this warning originates in the macro `ascent` (in Nightly builds, run with -Z macro-backtrace for more info)

warning: using `clone` on type `i32` which implements the `Copy` trait
 --> src/main.rs:9:13
  |
9 |    relation path(i32, i32);
  |             ^^^^
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#clone_on_copy
help: try removing the `clone` call
  |
9 ~    relation ascent! {
10+    relation edge(i32, i32);
11+    relation path(i32, i32);
12+       
13+    path(x, y) <-- edge(x, y);
14+    path(x, z) <-- edge(x, y), path(y, z);
15~ }(i32, i32);
  |

warning: field assignment outside of initializer for an instance created with Default::default()
  --> src/main.rs:17:4
   |
17 |    prog.edge = vec![(1, 2), (2, 3)];
   |    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
note: consider initializing the variable with `AscentProgram { edge: vec![(1, 2), (2, 3)], ..Default::default() }` and removing relevant reassignments
  --> src/main.rs:16:4
   |
16 |    let mut prog = AscentProgram::default();
   |    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#field_reassign_with_default
   = note: `#[warn(clippy::field_reassign_with_default)]` on by default

[s-arash]

Fair enough. With the litany of (not helpful) Clippy warnings, I may just silence all of the Clippy warnings for the generated code.

[jjl]

if they aren't giving any useful info, may as well.