Changes

Make generic signature/descriptor remapping methods static

In other projects it was a remarkably frequent usecase to remap
descriptors while not having a Remapper instance at hand. As technically
speaking only a MappingLookup instance is needed for this remapping
process, the methods have been turned static.

Some might complain about "static abuse", but realistically I've
always been asking why I didn't turn everything into a static method
from the get-go.

This change probably does not carry any performance difference,
but I haven't profiled it myself. As I also added a few null annotations,
you shouldn't be expecting the jar size to shrink even if we saved a few
opcodes from ALOAD 0; GETFIELD calls being reduced to ALOAD
 (commit: a2caecd)