Shaders: add gcore-shaders and make graster-nodes no-std

[Firestar99]

Requires #2924

• Extracts several symbols from gcore into gcore-shaders and reexports them in gcore
• moves anything related to ChoiceTypeStatic from registry to a separate choice_type mod
• Makes graster-nodes be no-std with an std feature to mask out node code

So you can now build graster-nodes in a no-std environment, a prerequisite for shaders:
cargo build -p graphene-raster-nodes --no-default-features

Why gcore-shaders

Simply put, there is still way too much stuff in gcore for it to become a no-std crate. So we either:

• create a new gcore-shaders crate that contains all of the no-std stuff, with very little masking needed
• turn gcore into no_std and move everything out of it
  • either I continue refactoring everything out of gcore into several crates, but that will eat up a lot of time, and runs the risk of no working shaders in the end
  • or we move everything to gnode, which would end up containing all the node definitions, registry and the like anyway. Stuff could get moved out from there later.
  • either way, a lot of migration needs to be hand-written
• mask out 99% of gcore, and you will forget to place the masks and break the build. Which is why I strongly prefer a separate crate.

[TrueDoctor]

I agree that it makes sense to have an actual no_std crate and we should have ci testing to confirm that it always builds without the std feature. We can still slim down gcore over time and let you focus on implementing gpu acceleration

[Firestar99]

There won't be any CI for the moment, but as soon as shaders work you need to compile shaders to compile graphite, and you'll have implicit CI for it being no_std.