Move codegen into separate crate

[stepancheg]

No description provided.

[jbowens]

I'd love to see no_std support from the crate that's imported by generated code at runtime. Separating codegen into a separate crate would be really helpful for that, so that codegen can freely depend on std.

[stepancheg]

codegen is now in separate crate (PR is about to land).

I'm going to create a separate issue for #[no_std], although I have no idea how #[no_std] works. Code should use only imports from core? Is Vec struct available? Is HashMap available? Should read and write be only to bytes, without using Read and Write traits? If you know how to do it, please tell me.