Mips support

This adds minimal support to process MIPS files. A very basic unittest is included.

When the mips executable is used for the readelf simulation test, it does show some issues where the python version of readelf doesn't quite follow binutils readelf. Some of these are already fixed in this patch set (ie. the PT_LOOS / SHT_LOOS output), but here are also issues with ra_regnum being shown in the dwarf sections where binutils doesn't.