MINOR: compiler: add FIXED_SIZE(size, type, name) macro

MINOR: compiler: add FIXED_SIZE(size, type, name) macro

FIXED_SIZE() macro can be used to instruct the compiler that the struct
member named <name>, handled as <type>, must be stored using <size> bytes
and that even if the type used is actualler smaller than the expected size

FIXED_SIZE_ARRAY(), similar to FIXED_SIZE() but for arrays: it takes an
extra argument which is the number of members.

They may be used for portability concerns to ensure a structure mapping
remains consistent between platforms.