riscv-macros: modularize code

[romancardenas]

This is part of the work for improving the procedural macros used in this repo.

I reduced to the minimum the amount of code in riscv-macros/src/lib.rs. The idea is leaving there only the public macros, and all the auxiliary code will be in clean modules.

Currently, riscv-macros only contain the pac_enum macro, which works in tandem with the riscv crate. Thus, the auxiliary code has been moved to the riscv module.

If the rt feature is active, pac_enum may also produce code required by riscv-rt. This additional code is now isolated in the riscv::rt module.

Next steps will be moving code from riscv-rt/macros to riscv-macros, under a new riscv_rt module.