Add `regalloc2-tool`

[Amanieu]

This PR has 2 components:

• A SerializableFunction which wraps a Function and MachineEnv. This type can be serialized and deserialized with Serde, and is enabled by the "enable-serde" feature.
• A regalloc2-tool binary which reads a bincode-encoded SerializableFunction and then runs the register allocator and checker on it.

This is a useful tool for debugging register allocation failures and to investigate cases of poor register allocation.

[Amanieu]

Here is an example of the output of this tool with the -v option which prints the function before and after register allocation. Note that this example is based on #131 since it requires 128 integer pregs.