Add OsStr methods for testing, stripping, and splitting Unicode prefixes.

[jmillikin]

ACP: rust-lang/libs-team#114

Discussion on that ACP seems to have quieted down without a clear consensus, so I'm going to throw this PR out for consideration. The general idea is that it's difficult to write prefix/suffix manipulations for OsStr that accept non-Unicode patterns[0], but the most pressing need (inspecting contents of args_os) are all based around Unicode patterns, and a big chunk of those operate exclusively on prefixes.

The first commit in this PR adds the OsStr::to_str_split() and OsString::into_string_split() methods, which extract the longest prefix from an OsStr / OsString that is valid Unicode. This prefix can then be parsed like normal, in platform-independent logic.

The second commit is based on discussion in the above ACP. I had considered this to be separate functionality, but there was some concern that extracting a Unicode prefix wasn't useful on its own, so I added some helper functions:

• OsStr::starts_with() tests whether an OsStr has a prefix matching the given Pattern.
  • It can be used as: if arg.starts_with("--") {
• OsStr::strip_prefix() returns the OsStr after removing a prefix matching the given Pattern.
  • It can be used as: if let Some(arg_value) = arg.strip_prefix("--some-flag=") {
• OsStr::split_once() splits an OsStr into a (&str, &OsStr) pair, where the delimiter matches a given Pattern.
  • It can be used as: let Some((flag_name, flag_value)) = arg.split_once("=") {

OsStr::starts_with_str() and OsStr::strip_prefix_str() are specialized variants that are implemented as &[u8] comparisons, which I expect to be more efficient than the Pattern versions since they don't need to validate UTF-8.

[0] See extensive prior discussions, for example rfcs/2295.

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[jmillikin]

@rustbot label +T-libs-api -T-libs

[Mark-Simulacrum]

r? @joshtriplett

[bors]

☔ The latest upstream changes (presumably #112624) made this pull request unmergeable. Please resolve the merge conflicts.