Only test files are modified.
Some tests for non-standard piece colors/types fail due to panics.
Some other small changes were done.

List of changes:
- Remove external dependencies, assert package.
- Add benchmarks to diag and game package
- Add benchmarks to perft suite in diag package.
  - Only first 10 tests from perftsuite.epd are used.
  - Depth 0 is omitted.
  - The BENCH_DEEP_PERFT_SUITE environment variable set to true, uses depth 5.
    WARNING: Can take a long time.
- Add benchmarks for games of various lengths to game package.
- Use-short flag to limit number and type of benchmarks.
- Fix "// Output" line in ExampleLegalMoves in game package to use the example
  in tests. It works now, because printing map is now sorted by keys and always passes the tests.
- Use package fen in game tests to decode boards in FEN format.
- Add tests and benchmarks to piece.
- Add tests for *Position.Find().
- Add tests and benchmark for *Position.Threatened().
- Add tests in TestRootMoves() with Black and non-standard colors as ActiveColor.
- Add tests and benchmark for *Position.Put().
- Add more tests for *Position.MakeMove().
- Add positionChanges type to track validate changes between moves in tests.
- Add test position changer function for castling rights.
- Add tests for *Position.Check().
- Add tests and benchmark for *Position.QuickPut().
- Add tests for *Position.MailBox().
- Add tests for *Position.MarshalJSON().

Some other changes were made to improve code readability and consistency.
For nicer test logs, *Position.String() is introduced.

List of changes:
- Fix *Position.Put() to pass the non-standard piece or square tests.
- Fix *Position.Threatened() panics on non-standard colors or squares.
- Fix *Position.Check() panics on non-standard colors.
- Fix *Position.QuickPut() panics on non-standard colors or squares.
- Add piece.OtherColor array and use where appropriate to increase readability
- Add comments to some local functions to warn that for some inputs a
  panic can be thrown.
- Add *Position.String() and tests
- Added makeMove positionChanger for testing purposes.
- Added clock positionChanger for testing purposes.
- Initial position example:
```
   a b c d e f g h
 ┌─────────────────┐
8│ r n b q k b n r │8
7│ p p p p p p p p │7
6│ . . . . . . . . │6
5│ . . . . . . . . │5
4│ . . . . . . . . │4
3│ . . . . . . . . │3
2│ P P P P P P P P │2
1│ R N B Q K B N R │1
 └─────────────────┘
   a b c d e f g h

MoveNumber: 1
ActiveColor: White
CastlingRights:
  White: O-O-O O-O
  Black: O-O-O O-O
EnPassant:
LastMove:
FiftyMoveCount: 0
ThreeFoldCount: 0
MovesLeft:
  White: 0
  Black: 0
Clocks:
  White: 0s
  Black: 0s
```

- Use `piece.COLOR_COUNT` instead of `2` for color in loops and array
  declarations to improve readability.
- Introduce and use `piece.TYPE_COUNT` constant.
- Change `piece.King+1` to `piece.TYPE_COUNT` in position tests.
- Use for-range loop instead for;; in *Position.Reset() for consistency.
- Make checks before accessing bitBoard color in *Position.Find(),
  *Position.Moves() to pass tests.
- New *Position.bitBoards() function which returns a BitBoards struct.
  Is used instead of 'BitBoards(p.bitBoard)' retyping, which is
  currently not possible.

- don't use map in piece.Type.String()
- don't use map in piece.Piece.Figurine()
- don't use map in piece.Color.String()