chore: remove unnecessary executable flags

Closes #4.

fix: use reg for procedural assignments

fix: lui, force rs1 zero, always add

feat: implement ebreak

Breaks the processor, can
exit the testcase

fix: jump according to zero flag, not LSB zero!!

feat: implement sb, sh, lb, lh support via masking

fix: do not set subtract for non-R instructions

fix: make immediates sign extended

fix: do not use immediate in alu src for SB

feat(decoder): implement memory mask, conditional jumps

feat: add instruction decoder