Implement conversion of test scripts to JavaScript

[rossberg]

Now that the interpreter is almost 0xc-complete, this should come in handy.

This patch extends the script language with a couple of meta facilities. The first is quoting of scripts (by wrapping them into (script <name>? ...)); this binds a script in the same way a module def binds a module. The second is the extension of the output command to .js target files, which converts the referenced script to an equivalent JS runner.

More importantly, you can access these facilities with simple command line switches. For example,

wasm -d script.wast -o script.js

will convert a test script to self-contained JavaScript, with all Wasm modules embedded as binary strings.

Caveat: All invoke's currently become calls directly from JavaScript, no module wrappers. Thus tests involving i64 params/results do not work yet. Will be addressed in a follow-up.

[Edit]

Extended to also support converting whole scripts to modified .wast files:

wasm -d script.wast -o script2.wast
wasm -d script.wast -o script2.bin.wast

When the output file extension is .bin.wast, then all embedded modules are represented as binary strings, otherwise textual. The only exception are modules in a newly introduced assert_malformed command, which retain their original format, since cross-coding is meaningless in that case.

The test runner is updated to use this functionality.

[rossberg]

Extended to also support cross-converting .wast scripts, such that all embedded modules become either textual or binary:

wasm -d script.wast -o script2.wast
wasm -d script.wast -o script2.bin.wast

Updated original comment.

[rossberg]

Any opposition to landing this and #341 ? Together these patches should enable using ml-proto directly for the same functionality as sexpr2wasm, especially for 0xc tests.

[titzer]

lgtm

[rossberg]

Conflicts resolved and merged.