Investigate impact of removing shell32 dependency of libstd on Windows

[alexcrichton]

An excellent blog post recently showed that removing the gdi32.dll dependency from LLVM speed up its test suite by a huge amount. For us libstd doesn't depend on gdi32.dll directly but it does depend on shell32.dll which depends on gdi32.dll (according to the blog post). Specificaly it looks like CommandLineToArgvW, the same function as LLVM, is used by us!

It'd be great to investigate removing the shell32 dependency from libstd (may be a few other functions here and there). We should run some tests on Windows to see if this improves rustc cycle times!

[retep998]

Based on my limited investigation, it's likely that replacing CommandLineToArgvW with a pure Rust equivalent should be good enough to get rid of the gdi32 dependency in things that only use libstd. So really someone just needs to write that pure Rust implementation which should be easy to do.

[Zoxc]

It would be nice to get rid of this in the compiler too. We spawn a lot of compiler instances during bootstrapping.

[retep998]

So now that the change has been implemented, have we actually investigated how much of an impact the change made?