ARM64 backend, part 3 / 11: MachInst infrastructure.

This patch adds the MachInst, or Machine Instruction, infrastructure.
This is the machine-independent portion of the new backend design. It
contains the implementation of the "vcode" (virtual-registerized code)
container, the top-level lowering algorithm and compilation pipeline,
and the trait definitions that the machine backends will fill in.

This backend infrastructure is included in the compilation of the
`codegen` crate, but it is not yet tied into the public APIs; that patch
will come last, after all the other pieces are filled in.

This patch contains code written by Julian Seward <jseward@acm.org> and
Benjamin Bouvier <public@benj.me>, originally developed on a side-branch
before rebasing and condensing into this patch series. See the `arm64`
branch at `https://github.com/cfallin/wasmtime` for original development
history.

Co-authored-by: Julian Seward <jseward@acm.org>
Co-authored-by: Benjamin Bouvier <public@benj.me>

Cargo.lock

@@ -379,6 +379,7 @@ dependencies = [
  "gimli",
  "hashbrown 0.7.1",
  "log",
+ "regalloc",
  "serde",
  "smallvec",
  "target-lexicon",
@@ -1599,6 +1600,16 @@ dependencies = [
  "rust-argon2",
 ]
 
+[[package]]
+name = "regalloc"
+version = "0.0.17"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "89ce0cd835fa6e91bbf5d010beee19d0c2e97e4ad5e13c399a31122cfc83bdd6"
+dependencies = [
+ "log",
+ "rustc-hash",
+]
+
 [[package]]
 name = "regex"
 version = "1.3.6"
@@ -1663,6 +1674,12 @@ version = "0.1.16"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "4c691c0e608126e00913e33f0ccf3727d5fc84573623b8d65b2df340b5201783"
 
+[[package]]
+name = "rustc-hash"
+version = "1.1.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "08d43f7aa6b08d49f382cde6a7982047c3426db949b1424bc4b7ec9ae12c6ce2"
+
 [[package]]
 name = "rustc_version"
 version = "0.2.3"