Raw buffer access for paths

[emberian]

In #7131 it becomes useful to have access to a raw buffer which is the platform's representation of the path for passing to system APIs. I propose a with_platform_buffer() method which returns the appropriate raw buffer. On Windows this would be UTF-16, elsewhere it'd just be bytes.

This could avoid doing allocations in lowlevel APIs which directly pass paths to the system.

[catamorphism]

Seems like a good idea to me.

[catamorphism]

I think this is addressed with the Path rewrite (e.g. ToCStr). Closing; reopen if there's something specific that's not there already.

[emberian]

Yup, the Path rewrite addressed this. Yay progress!