Initial draft of DSL semantics complete.

This latest refactor adds "extractor macros" in place of the
very-confusing-even-to-the-DSL-author reverse-rules-as-extractors
concept. It was beautifully symmetric but also just too mind-bending to
be practical.

It also adds argument polarity to external extractors. This is inspired
by Prolog's similar notion (see e.g. the "+x" vs. "-x" argument notation
in library documentation) where the unification-based semantics allow
for bidirectional flow through arguments. We don't want polymorphism
or dynamism w.r.t. directions/polarities here; the polarities are
static; but it is useful to be able to feed values *into* an extractor
(aside from the one value being extracted). Semantically this still
correlates to a term-rewriting/value-equivalence world since we can
still translate all of this to a list of equality constraints.

To make that work, this change also adds expressions into patterns,
specifically only for extractor "input" args. This required quite a bit
of internal refactoring but is only a small addition to the language
semantics.

I plan to build out the little instruction-selector sketch further but
the one that is here (in `test3.isle`) is starting to get interesting
already with the current DSL semantics.

cranelift/isle/isle_examples/test.isle

@@ -3,7 +3,7 @@
 (type B (enum (B1 (x u32)) (B2 (x u32))))
 
 (decl Input (A) u32)
-(extractor Input get_input) ;; fn get_input<C>(ctx: &mut C, ret: u32) -> Option<(A,)>
+(extern extractor Input get_input) ;; fn get_input<C>(ctx: &mut C, ret: u32) -> Option<(A,)>
 
 (decl Lower (A) B)
 
@@ -12,10 +12,10 @@
   (B.B2 sub))
 
 (decl Extractor (B) A)
-(rule
-  (A.A2 x)
-  (Extractor (B.B1 x)))
+(extractor
+  (Extractor x)
+  (A.A2 x))
 
 (rule
   (Lower (Extractor b))
-  b)
+  (B.B1 b))