(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

(Third try re: #1819 and #1820.)

[brson]

I pushed it anyway because I'm not that concerned about the performance of starts_with, but if we can avoid traversing the entirety of both strings that would be better.