[spec] Introduce multi-results and block parameters

[rossberg]

This does all changes to the spec necessary to support multi-results and block parameters:

• Replaces all occurrences of result types [t?] with [t*]
• Extends the syntax of block signatures to allow for a type index that denotes a function type
• Generalises the validation rules for block/loop/if to allow block params
• Generalises the exectuion rules for block/loop/if to allow block params
• Extends the binary format with type indices as block sigs
• Extends the text format with typeuse and param decls (which must not contain identifiers)
• Removes respective notes about future extensions

Most of these changes are very simple. The main complication ironically is the mere representation of block sigs in the syntax and binary format, which necessarily becomes a bit ad-hoc because it has to support both the old and the new format. Moreover, interpreting a block type may now require a lookup in the type section, which is a minor structural complication in the respective rules.

Preview at https://webassembly.github.io/multi-value/

Plenty of tests and a respective patch for the interpreter are in PR #2.

[lukewagner]

Pre-existing, but accentuated by binary format changes to blocktype: to clarify the "must not collide with" relationship between the byte codes for the valtypes, anyfunc, function ctor, and ε, it'd be useful to collect them into a single table (perhaps like you've done with the instructions). E.g., in SM we have a TypeCode enum that is then used to define ValType etc.

In general, blocktype changes makes sense from what I scanned.

[rossberg]

@lukewagner, added type index as part of WebAssembly/spec#526.

[lukewagner]

Beautiful, thanks!