Support C-style enums with more than 256 variants.

[finnbear]

Fixes #45 🎉

• Remove unsafe cast of CowSlice
• Remove histogram for C_STYLE enum's
• Ensure missing repr is zero-cost
• check_less_than

test derive::variant::test2::bench_enum_300_variants_vec_decode ... bench:         159 ns/iter (+/- 8)
test derive::variant::test2::bench_enum_300_variants_vec_encode ... bench:         262 ns/iter (+/- 414)
test derive::variant::tests::bench_bool_enum_vec_decode         ... bench:         140 ns/iter (+/- 15)
test derive::variant::tests::bench_bool_enum_vec_encode         ... bench:         359 ns/iter (+/- 19)