[cage] improve IMCC_debug_ins for headerizer

the odd function format was not picked up by the headerizer
add pod also

INLINE to imc.h added by headerizer
[cage] shim unused arguments in src/io/*.c

silence c++ compiler warnings
[cage] key.pmc minor optimization

avoid default case integer key for get_type to reduce code size.
numbers cannot be keys.

better UNIMPLEMENTED error messages