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PIC24/30/33 addressing modes

Assembler addressing modes.  The problem with the assembler syntax as
specified by Microchip is that it uses a syntax that's not so easily
expressed as s-expressions.  However, addressing modes really are just
names mapped to bit representation so they might be handled as
constants.  I'm just not quite sure about how this will work with
transformation / optimization.

So, let's just try it: setup the infrastructure (assembler + stub
compiler interface) and fill it in.

Next: implement 'drop' as "MOV [--W14], W0".  The encoding:

01111 wwww B hhh dddd ggg ssss

w: offset
B: byte mode
h: dst address mode
d: dst reg
g: src address mode
s: src reg

The problem here is that an s-expression syntax would probably need to
provide a bit of sugar, since the instruction format itself is quite
spartan.  In the current framework, an instruction is a function with
a number of binary inputs.  This needs to change to something more
general, preferably representable as combinators.

Also, `MOV' is used for a lot of different instruction tags.

Addressing modes:
000 Ws      ;; Register direct
001 [Ws]    ;; Indirect
010 [Ws--]  ;; .. post-dec
011 [Ws++]  ;; .. post-inc
100 [--Ws]  ;; .. pre-dec
101 [++Ws]  ;; .. pre-inc
11x [Ws+Wb] ;; Offset / Unused (RESET)

Proposed s-expr syntax:

Ws
(* Ws)
(*-- Ws)
(*++ Ws)
(--* Ws)
(++* Ws)


The troubling element is offset addressing.  Instead of representing
it as (+ Wb Ws) it needs to not refer to offset register, as this is a
global entitiy.



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