Implement parsing of tables with initializer expressions

[yurydelendik]

The function references proposal adds initializer for tables when its type is non-nullable reference. The text format is extended with constant expression (which can have ref.func). The binary format adds 0x40 0x00 prefix to the table section entry to indicate expression presence.

https://github.com/WebAssembly/function-references/blob/main/proposals/function-references/Overview.md

[yurydelendik]

See also https://bugzilla.mozilla.org/show_bug.cgi?id=1784499

[alexcrichton]

Thanks! Reading over the proposal though I'm not sure if the TableType is the best place to store and parse this. It seems that the initialization expression is only a matter for table definitions, not general type declarations such as table imports. If that's the case, could the parsing here be updated to only parse the initialization expression for a table declaration rather than a general table type?

[yurydelendik]

could the parsing here be updated to only parse the initialization expression for a table declaration rather than a general table type?

I made this decision based on implied idea that this default/init expression becomes a "metadata" that affect other operations such as table.grow.

[alexcrichton]

I don't think that it's type level information, though, but rather runtime information? I could be misunderstanding the spec, though, but for example when a table is imported is the initializer element specified? If so I would expect that the spec would grow more logic around the "matching" semantics to test whether two table types are equivalent, and if not I think it's runtime information that is stateful but not part of the type information.

[yurydelendik]

Simplified patch by moving init_expr out of TableType.