x64: move encodings to a separate module

In order to benchmark the encoding code with criterion, the functions
and structures must be public. Moving this code to its own module
(instead of keeping as a submodule to `inst`), allows `inst` to remain
private. This avoids having to expose and document (or ignore
documenting) the numerous instruction variants in `inst` while allowing
access to the encoding code. This commit changes no functionality.

cranelift/codegen/src/isa/x64/mod.rs

@@ -18,6 +18,7 @@ use target_lexicon::Triple;
 use crate::isa::unwind::systemv;
 
 mod abi;
+pub mod encoding;
 mod inst;
 mod lower;
 mod settings;