Remove bit_field dependency

[romancardenas]

I replaced the bit_field crate with bit-wise operations. In this way, we avoid needless checks for getting bits from the registers. Now, future versions of the riscv crate will be slightly faster (and smaller)

[romancardenas]

ping @rust-embedded/risc-v !

If you prefer, I can add private getter and setter methods to the register macros to make the code look "cleaner", but I think it is OK as is.

[romancardenas]

ping @rust-embedded/riscv @rust-embedded/risc-v

[almindor]

Hey sorry for the delay, I've been really busy lately. I'll try to review this ASAP.