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/encoding/mod.rs

@@ -0,0 +1,60 @@
+//! Contains the encoding machinery for the various x64 instruction formats.
+use crate::{isa::x64, machinst::MachBuffer};
+use std::vec::Vec;
+
+pub mod evex;
+pub mod rex;
+pub mod vex;
+
+/// The encoding formats in this module all require a way of placing bytes into
+/// a buffer.
+pub trait ByteSink {
+    /// Add 1 byte to the code section.
+    fn put1(&mut self, _: u8);
+
+    /// Add 2 bytes to the code section.
+    fn put2(&mut self, _: u16);
+
+    /// Add 4 bytes to the code section.
+    fn put4(&mut self, _: u32);
+
+    /// Add 8 bytes to the code section.
+    fn put8(&mut self, _: u64);
+}
+
+impl ByteSink for MachBuffer<x64::inst::Inst> {
+    fn put1(&mut self, value: u8) {
+        self.put1(value)
+    }
+
+    fn put2(&mut self, value: u16) {
+        self.put2(value)
+    }
+
+    fn put4(&mut self, value: u32) {
+        self.put4(value)
+    }
+
+    fn put8(&mut self, value: u64) {
+        self.put8(value)
+    }
+}
+
+/// Provide a convenient implementation for testing.
+impl ByteSink for Vec<u8> {
+    fn put1(&mut self, v: u8) {
+        self.extend_from_slice(&[v])
+    }
+
+    fn put2(&mut self, v: u16) {
+        self.extend_from_slice(&v.to_le_bytes())
+    }
+
+    fn put4(&mut self, v: u32) {
+        self.extend_from_slice(&v.to_le_bytes())
+    }
+
+    fn put8(&mut self, v: u64) {
+        self.extend_from_slice(&v.to_le_bytes())
+    }
+}