Fixed debug logging in static struct/bitfield initialisation
Redirect asm_reserve_bytes through asm_declare_pad

This prevents ".space 0" directives
Added bitfield padding skip-init test
Fixed zero width bitfields affecting struct offsetting/size
Also set struct_offset for 0-length bitfields
Fixed bitfield parsing after comma
Comment + note about bitfield-pad initialisation
Always initialise anonymous bitfields - backend handles it
Don't initialise 0-length bitfields
Moved operator const-eval to const.[ch]
Fixed constant op dropping in backend
Backend constant folding (e.g. for bitfield masking)

Also tidied code flow for out_op()
out/asm.c tidy up
Backend static-struct output with better bitfield padding
Don't output any static storage for empty bitfields
Test for bitfield init/padding skip
Bitfield union initialisation tests
Bitfield end-padding init test
Return 0 on 0_width test