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/test4.isle

@@ -3,8 +3,8 @@
 
 (decl Ext1 (u32) A)
 (decl Ext2 (u32) A)
-(extractor Ext1 ext1)
-(extractor Ext2 ext2)
+(extern extractor Ext1 ext1)
+(extern extractor Ext2 ext2)
 
 (decl Lower (A) A)