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/isa/intel/enc_tables.rs

@@ -2,7 +2,8 @@
 
 use ir::InstructionData;
 use ir::types;
-use isa::enc_tables::{Level1Entry, Level2Entry};
+use isa::EncInfo;
 use isa::constraints::*;
+use isa::enc_tables::{Level1Entry, Level2Entry};
 
 include!(concat!(env!("OUT_DIR"), "/encoding-intel.rs"));