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>

cranelift/codegen/Cargo.toml

@@ -24,6 +24,7 @@ gimli = { version = "0.20.0", default-features = false, features = ["write"], op
 smallvec = { version = "1.0.0" }
 thiserror = "1.0.4"
 byteorder = { version = "1.3.2", default-features = false }
+regalloc = "0.0.17"
 # It is a goal of the cranelift-codegen crate to have minimal external dependencies.
 # Please don't add any unless they are essential to the task of creating binary
 # machine code. Integration tests that need external dependencies can be
@@ -33,7 +34,7 @@ byteorder = { version = "1.3.2", default-features = false }
 cranelift-codegen-meta = { path = "meta", version = "0.62.0" }
 
 [features]
-default = ["std", "unwind"]
+default = ["std", "unwind", "all-arch"]
 
 # The "std" feature enables use of libstd. The "core" feature enables use
 # of some minimal std-like replacement libraries. At least one of these two