Clarify and regularize text format index parsing rules

[lukewagner]

This PR intends to resolve #648 by regularizing and more-clearly defining the text format rules for core- and component-level indices, identifiers and inline aliases. One nice thing with the changes in this PR is that there is now an explicit core-prefix(...) in the grammar at all the places where core might (when a plain $id is not used) get injected.

The test/syntax/indices.wast tries to test all the syntactic cases touched by this PR. The commented-out lines currently fail in wasm-tools and the rest pass as-is. This helps identify what the concrete delta from the status quo is. I didn't add any negative tests since I expect we'll need a phase-out process before rejecting anything that is accepted now.

PTAL @bvisness @alexcrichton

[alexcrichton]

Mind skimming over and double-checking some of the test-related changes on bytecodealliance/wasm-tools#2562?

[keithw]

Seeing this via the PR in wasm-tools; sorry for the late comment. If the component-model text format is being regularized, is this a good time to clarify the ordering requirements for symbolic identifiers?

In core Wasm, the use of an identifier can come before the identifier is bound:

(module
  (export "" (func $f))
  (func $f)
)

but this seems to be malformed in a component, in the sense that wasmparser's Parser rejects it (but I don't think there is explicit discussion of this in the Explainer yet or an assert_malformed test case):

(component
  (export "" (core module $m))
  (core module $m)
)

To me the more vexing case is something like:

(component
  (instance (export "" (instance 1)))
  (instance $i)
)

... which is accepted by wasmparser's Parser, but wasmprinter prints it out as:

(component
  (instance (;0;)
    (export "" (instance $i))
  )
  (instance $i (;1;))
)

which is rejected by wasmparser the next time (both before and after bytecodealliance/wasm-tools#2562).

tl;dr If attention is going into component WAT at the moment, is this the right time to lock down what is considered well-formed? This could help get to a place where all well-formed components successfully roundtrip through wasmparser->wasmprinter->wasmparser. (We got there for core modules in bytecodealliance/wasm-tools#2148 .)