File stale path references.

After rearranging the directory layout, some paths in documentation
needed updating.

Fix some typos too.

lib/cretonne/src/isa/enc_tables.rs

@@ -1,7 +1,7 @@
 //! Support types for generated encoding tables.
 //!
 //! This module contains types and functions for working with the encoding tables generated by
-//! `meta/gen_encoding.py`.
+//! `lib/cretonne/meta/gen_encoding.py`.
 use ir::{Type, Opcode};
 use isa::Encoding;
 use constant_hash::{Table, probe};