(core::str) mark push/pop/shift byte functions unsafe, rename len functions, and fix replace

[killerswan]

Various updates to core::str, including:

• move push_byte, push_bytes, pop_byte, and shift_byte into str::unsafe and mark them unsafe
• rename byte_len -> len_bytes
• rename char_len -> len / len_chars
• rename byte_len_range -> substr_len_bytes and
• rename char_len_range -> substr_len_chars
• fix replace and starts_with (for UTF-8) and add more unicode find/contains/replace test cases

[brson]

This has a weird duplicate of an existing commit: 09b2f8d

[killerswan]

I need to figure out how to reset my master branch, after some accidental rebasing...