[spec/interpreter/test] Implement basic tail-call proposal

[rossberg]

This PR specs, implements, and tests a basic proposal for tail calls:

• It adds instructions return_call and return_call_indirect.
• Spec for validation, execution, binary, text format
• Implementation in the interpreter
• Tests for both instructions

The PR does not yet introduce any typing distinction for functions that are tail-callers or callees, the details of which still need to be worked out.

Rendered spec is at https://webassembly.github.io/tail-call/core