feat: implement bytes type

[thesayyn]

• Value::to_str() now calls collect_str() instead of falling back to to_repr(), so print(b"hello")
  outputs hello instead of b"hello"
• ord() accepts 1-byte bytes per spec — ord(b"A") == 65; returns the byte value as an integer
• StarlarkBytes is now part of the public API — starlark::values::bytes::StarlarkBytes is accessible to
  downstream crates
• AllocValue/AllocFrozenValue impls for &[u8] and Vec — heap.alloc(b"hi" as &[u8]) and
  heap.alloc(vec![1u8, 2]) now work directly
• StarlarkBytes::from_vec() constructor converts Vec to Box<[u8]> in-place, eliminating a redundant
  copy in add, mul, slice, and bytes()
• Duplicate bytes key detection in the linter — {b"a": 1, b"a": 2} now raises a duplicate-key warning

[meta-codesync]

@facebook-github-bot has imported this pull request. If you are a Meta employee, you can view this in D95696965. (Because this pull request was imported automatically, there will not be any future comments.)

[ndmitchell]

Thanks! I landed this as 10c5862 and 69593bb. Really high quality diff! Very nice work.

[thesayyn]

thank you