Types and other issues with cgen

[Doug Evans]

Doug Evans writes:
 >  > Michael writes
 >  > 2) Cgen has a type mechanism (DI/SI/etc.) but it doesn't seem to be used in the
 >  >    actual code for at least the assembler and disassembler
 > 
 > Using the modes in the assembler/disassembler isn't the right way to go.
 > These modes are for semantic operation, not assembly/disassembly.
 > Imagine some instruction with an immediate operand that is a fixed
 > set of constants that is encoded with special magic numbers.
 > Register indices are another example.
 > There's a disconnect between representation in the instruction
 > and use during semantic evaluation.

Blech.  I should have let my reply sit in my head awhile before
^c^c-ing.

Here's my proposal.  We record in cgen_fields a type big enough
to hold the field (or some minor variant thereof) and have
inserters/extractors for each type in use.