Intent to prototype: WebAssembly tail calls

[Lars Hansen]

Summary: Add proper tail call instructions to our WebAssembly implementation

Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1571996

Standard: https://github.com/WebAssembly/tail-call

Platform coverage: All

Preference: TBD

Devtools: N/A

Other browsers: Available in Chrome behind #enable-experimental-webassembly-features

Tests: Reference tests will appear in WPT in the normal manner when the feature is standardized; there will be additional jit-tests, in development (js/src/jit-test/tests/wasm/tail-calls)

Spec stability: stable, apart from a detail about inter-instance tail calls which we hope to resolve through our prototyping effort

--lars