Cursor library.

Add a new cursor module and define an EncCursor data type in it. An
EncCursor is a cursor that inserts instructions with a valid encoding
for the ISA. This is useful for passes generating code after
legalization.

Implement a builder interface via the new InstInserterBase trait such
that the EncCursor builders support with_result().

Use EncCursor in coalescing.rs instead of the layout cursor as a proof
of concept.

lib/cretonne/src/lib.rs

@@ -34,6 +34,7 @@ pub mod verifier;
 mod abi;
 mod constant_hash;
 mod context;
+mod cursor;
 mod iterators;
 mod legalizer;
 mod licm;