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/src/verifier/mod.rs

@@ -697,11 +697,10 @@ impl<'a> Verifier<'a> {
                                     isa.encoding_info().display(encoding));
                     }
                 }
-                Err(e) => {
+                Err(_) => {
                     return err!(inst,
-                                "Instruction failed to re-encode {}: {:?}",
-                                isa.encoding_info().display(encoding),
-                                e)
+                                "Instruction failed to re-encode {}",
+                                isa.encoding_info().display(encoding))
                 }
             }
             return Ok(());