Moves the instrlist_insert_{mov,push}_{immed_ptrsz,instr_addr}() utility routines from instrument.c to instrlist.c for inclusion in drdecode. Moves their first-level helpers from mangle_shared.c to a new file mangle_utils.c. Makes their second-level helpers inside arch//mangle.c exposed to drdecode.
Adds minimal tests to api.drdecode.
Fixes #3100 (closed)