Support for reference types

[bakkot]

The reference types proposal includes some new instructions not supported in this grammar.

Concretely, I believe the missing things are

• externref and funcref as value types
• funcref as a value type (it was already a legal table type)
• new instructions for working with references: ref.null externref|funcref, ref.is_null, and ref.func funcidx
  • ref.null example
  • ref.is_null example
  • ref.func example
• new instructions for working with tables
  • not going to link examples of them all individually, but the table_* tests in this directory cover them
• a change to call_indirect to support table indexes
  • e.g.
• a change to select to allow select (result $type)
  • e.g.

I started writing a PR for this, but I'm new to Sublime grammars and got bogged down in some of the details and complexity, so I'm opening this issue instead.