Remove C backend infrastructure

Issue #384
Remove codegen and icode infrastructure

Issue #384
Remove old tests specific to the C backend
Remove old tests specific to the C backend

Issue #384
PEP 8 fixes for checkexpr.py
Missed an error in checkexpr.py
PEP 8 fixes for checkmember.py
PEP 8 fixes for coerce.py
PEP8 fixes for compilemapd.py
PEP 8 fixes for compileslotmap.py
PEP 8 fixes for constraints.py
PEP 8 fixes for erasetype.py
PEP 8 fixes for errors.py
PEP 8 fixes for expandtype.py
PEP 8 fixes for icode.py
PEP 8 fix for infer.py
PEP 8 fixes for join.py
PEP 8 fixes for lex.py
PEP 8 fixes for maptypevar.py
PEP 8 fixes for meet.py