Return a function pointer from TargetIsa::encode().

Replace the isa::Legalize enumeration with a function pointer. This
allows an ISA to define its own specific legalization actions instead of
relying on the default two.

Generate a LEGALIZE_ACTIONS table for each ISA which contains
legalization function pointers indexed by the legalization codes that
are already in the encoding tables. Include this table in
isa/*/enc_tables.rs.

Give the `Encodings` iterator a reference to the action table and change
its `legalize()` method to return a function pointer instead of an
ISA-specific code.

The Result<> returned from TargetIsa::encode() no longer implements
Debug, so eliminate uses of unwrap and expect on that type.

lib/cretonne/meta/gen_encoding.py

@@ -875,7 +875,7 @@ def gen_isa(isa, fmt):
     emit_recipe_sizing(isa, fmt)
 
     # Finally, tie it all together in an `EncInfo`.
-    with fmt.indented('pub static INFO: EncInfo = EncInfo {', '};'):
+    with fmt.indented('pub static INFO: isa::EncInfo = isa::EncInfo {', '};'):
         fmt.line('constraints: &RECIPE_CONSTRAINTS,')
         fmt.line('sizing: &RECIPE_SIZING,')
         fmt.line('names: &RECIPE_NAMES,')