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/reader/src/parser.rs

@@ -108,7 +108,8 @@ impl<'a> Context<'a> {
     fn find_recipe_index(&self, recipe_name: &str) -> Option<u16> {
         if let Some(unique_isa) = self.unique_isa {
             unique_isa
-                .recipe_names()
+                .encoding_info()
+                .names
                 .iter()
                 .position(|&name| name == recipe_name)
                 .map(|idx| idx as u16)