Library::open_already_loaded() for Windows

[YaLTeR]

Fixes the Windows half of #27.

For unix I guess I can make a cfg-gated function? Not sure which platforms though. Also, if so, should I also add a cfg-gated top-level Library function?

[nagisa]

For unix I guess I can make a cfg-gated function?

So far I have avoided adding cfg-gated APIs that are not obviously non-portable at a glance. use os::platform makes it obvious for unix vs windows non-portability and is the same scheme that the standard library uses so it is familiar to both developers and reviewers. It would be a shame if we had an API in os::unix that didn't work for all unix targets.

There's an RFC for portability lint for a more general and less involving approach, but it hasn't been implemented. Until it, or something similar to it, is implemented, libloading should probably refrain from adding such APIs.

While we're waiting users of the library can still grab RTLD_NOLOAD from libc and pass it to os::unix::Library::open.

[YaLTeR]

So, any other changes? Bikeshedding for the method name?

[nagisa]

Ah yeah, thanks for the ping, I had forgotten about this entirely!

[YaLTeR]

Cheers!