Add the concept of "backend intrinsics" to Cranelift

[alexcrichton]

This commit is an initial stab at implementing interpreter-to-host communication in Pulley. The basic problem is that Pulley needs the ability to call back into Wasmtime to implement tasks such as memory.grow, imported functions, etc. For native platforms this is a simple call_indirect operation in Cranelift but the story for Pulley must be different because it's effectively switching from interpreted code to native code.

The solution I've ended up settling on looks pretty similar to native platforms but with a few important tweaks:

• A new call_indirect_host opcode is added to Pulley.
  • Function signatures that can be called from Pulley bytecode are statically enumerated at build-time.
  • This enables the implementation of call_indirect_host to take an immediate of which signature is being used and cast the function pointer to the right type.
• A new "backend intrinsic" concept is added to Cranelift.
  • This is a new variant of ExternalName.
  • The intention is that this has backend-specific meaning.
  • For Pulley, this means that the Nth function signature is being called.
• Code generation for Pulley in wasmtime-cranelift now has Pulley-specific handling of the wasm-to-host transition where all previous call_indirect instructions are replaced with a call to a "backend intrinsic" which gets lowered to a call_indirect_host.

Note that most of this still isn't hooked up everywhere in Wasmtime. That means that the testing here is pretty light at this time. It'll require a fair bit more work to get everything fully integrated from Wasmtime in Pulley. This is expected to be one of the significant remaining chunks of work and should help unblock future testing (or make those diffs smaller ideally).

[alexcrichton]

cc @cfallin as you're also no doubt interested in this as well

[cfallin]

LGTM on the Cranelift bits -- happy to dive deeper into the Pulley and Wasmtime bits too if you'd like but I'm not as deep into that context at the moment.

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[bjorn3]

Instead of adding a whole new instruction, would it be possible to use a UserExternalName with a reserved namespace like u32::MAX? Or the Pulley interpreter could have a set of UserExternalName for which to call the specified host function without reserving any namespace.