Create a mkNamedVar and mkNamedRef to allow giving a base name for variables
and references.
Make the state monad carry a VarId instead of an integer.
Allocate uniques and put them in variable names.
Use the newly created mkNamedVar instead of creating raw variables.
Remove the VariableRole datatype.
Merge pull request #64 from pjonsson/retire-varrole

Retire VariableRole

Conflicts:
	lib/Feldspar/Compiler/Imperative/FromCore.hs