Reject duplicate field names in structs

[stevenfontanella]

Part of #8261. Partially fixes struct.wast spec test.

[kripken]

lgtm otherwise

[tlively]

What kind of round tripping are we talking about here?

[stevenfontanella]

In wasm-dis when trying to produce the text from the binary, we try to print an empty name and crash:

Running with 128 workers
wasm-dis: /usr/local/google/home/stevenfont/code/binaryen/src/support/name.cpp:44: std::ostream& wasm::Name::print(std::ostream&) const: Assertion `*this && "Cannot print an empty name"' failed.
Aborted spec test suite execution after first failure. Set --no-fail-fast to disable this.
Failed tests:
.. struct-upstream.wast
executing:  /usr/local/google/home/stevenfont/code/binaryen/bin/wasm-shell /usr/local/google/home/stevenfont/code/binaryen/test/spec/struct-upstream.wast
        testing split module 0
executing:  /usr/local/google/home/stevenfont/code/binaryen/bin/wasm-opt test-spec-struct-upstream_split0.wast -O -all -q
         (binary format check)
             /usr/local/google/home/stevenfont/code/binaryen/bin/wasm-as test-spec-struct-upstream_split0.wast -o test-spec-struct-upstream-a.wasm -all -g
             /usr/local/google/home/stevenfont/code/binaryen/bin/wasm-dis test-spec-struct-upstream-a.wasm -o test-spec-struct-upstream-ab.wast -all
Command '['/usr/local/google/home/stevenfont/code/binaryen/bin/wasm-dis', 'test-spec-struct-upstream-a.wasm', '-o', 'test-spec-struct-upstream-ab.wast', '-all']' died with <Signals.SIGABRT: 6>.

A smaller repro is by roundtripping this:

(module
  (type (struct (field $x i32)))

  (func (param (ref 0)) (result i32) (struct.get 0 $x (local.get 0)))
)

AFAICT there's no logic for printing type indices instead of names when the type doesn't have a name, we'd need to add that.

[tlively]

Wow! Good thing we have spec tests :)