Move encoding-related information into an EncInfo struct.

The tables returned by recipe_names() and recipe_constraints() are now
collected into an EncInfo struct that is available from
TargetIsa::encoding_info(). This is equivalent to the register bank
tables available fro TargetIsa::register_info().

This cleans of the TargetIsa interface and makes it easier to add
encoding-related information.

lib/cretonne/src/write.rs

@@ -184,7 +184,7 @@ fn write_instruction(w: &mut Write,
     if let Some(enc) = func.encodings.get(inst).cloned() {
         let mut s = String::with_capacity(16);
         if let Some(isa) = isa {
-            write!(s, "[{}", isa.display_enc(enc))?;
+            write!(s, "[{}", isa.encoding_info().display(enc))?;
             // Write value locations, if we have them.
             if !func.locations.is_empty() {
                 let regs = isa.register_info();