More string fixes

[killerswan]

This checks various things off my TODO list, including:

• Rename str::from_byte(s) to str::unsafe::from_byte(s), mark them as unsafe, make comp/driver/driver.rs use str::from_bytes...
• Rename str::loop_chars to str::all, rename str::loop_chars_sub to str::substr_all, and propagate this change to std::rope and rustdoc's calls to these
• Rename str::to_chars -> str::chars
• Rename str::iter_chars -> str::chars_iter
• Add str::split_chars_iter and str::splitn_chars_iter
• Re-implement str::to_upper and str::to_lower using str::map

[brson]

Merged.