Add NO_STD switch

[arkkors]

Adds the NO_STD switch to Corrosion, allowing building
no_std crates in CMake without linking additional libraries.

Closes #153

[ogoffart]

Looks good!

I just wonder if the option should not be called just NO_STD to match the rust attribute.
Or is that a different thing?
Does it work for a no_std crate that uses the alloc crate?

[arkkors]

1. Yeah, NO_STD is shorter :) Also, I don't really see the distinction (at least in practice).

2. I don't see why it shouldn't work with alloc, but I'm not entirely sure. Will try that out later.

[arkkors]

1. The nostd test should now work under 1.44.1, so the test suite should (hopefully) pass now.
2. Regarding the alloc crate question: The alloc crate itself works in this setup. You need to define a global allocator though and probably setup an alloc error handler (e.g via #![feature(default_alloc_error_handler)]). This needs a nightly toolchain AFAIK. But it works :)

[jschwe]

Thanks for the PR. I see you also touched gen_cmake/target.rs. If possible, could you also add the same changes (basically add the include_platform_libs switch) to the experimental generator in at cmake/CorrosionGenerator.cmake? Should be this function: https://github.com/corrosion-rs/corrosion/blob/master/cmake/CorrosionGenerator.cmake#L25

[arkkors]

Here you go. Not very pretty, but it works.