with_ascii_lowercased zig builtin

[HajagosNorbert]

Please, before you approve, check out the zig code changes, since I'm unsure about handling reference counts. Tests were passing whether I put a referece count decrement statement in the with_ascii_lowercased function or not for the non-unique strings branch, so the string.decref(); is in there quiet arbitrarily.

initFromBigStr isn't used anywhere, so I deleted it.

The test with_ascii_lowercased_non_zero_refcounttest in crates/compiler/test_gen/src/gen_str.rs is an attempt at testing the function when the refcount of it's input string is > 1, but I'm not sure if that's what it does actually.

[HajagosNorbert]

CI was failing on the roc formatter check and on the use of camelCased Str.dropPrefix in one of the test.
I don't know why the macos-apple-silicon job ran from > 1h.