Fix imm coding in B type

Merge pull request #5 from rswarbrick/jal

Fix encoding for J-type instructions

Merge pull request #4 from rswarbrick/shifts

Fix import of TerminateException in sim.py

Merge pull request #3 from rswarbrick/shifts

Fixes for SRL and SRA

Fix variant extension handling

Smaller fixes and add variant to decode

Allow to write verbose trace to file

Further fixes to instruction coding

Fixes to support encoding/decoding

Generalize decode and encode

This probably needs some rework