Add mechanism for displaying the WAT signatures of WASI modules

[abrown]

@pchickey, here are the beginnings of an idea for displaying signatures of WITX interfaces. What I need is some way to easily determine what WASI modules will look like after the WITX is translated away. I felt like WAT was a good format for this, though ideally I would want to generate the same type of information in other languages (e.g. as C headers or AssemblyScript declarations).

Currently, when I run cargo run --example witx -- wat-signature ../../phases/ephemeral/witx/wasi_ephemeral_nn.witx, this PR outputs something like the following:

(module
 (import "wasi_ephemeral_nn" "load" (func (param I32 I32 I32 I32 I32) (result I32)))
 (import "wasi_ephemeral_nn" "init_execution_context" (func (param I32 I32) (result I32)))
 ...
)

I am seeking feedback on whether this type of helper command is useful to others before I invest more time to fix it up (e.g. use a proper WAT library, add comments, fix type casing, etc.).

[pchickey]

This looks useful and I'm glad that its a very compact and easy thing to implement so far. I don't have a strong opinion about whether it should use a library to help render it - the current witx sexpr-rendering code does not, and probably should. Thank you for doing this!

[abrown]

the current witx sexpr-rendering code does not, and probably should

Is this the ideal way to proceed? E.g. I see that SExpr implements Display--should I build up an sexpr for each signature and print those?

[pchickey]

That is the "good enough" solution I got working, but its still a pretty unpleasant output for a human to read because it doesn't try to put line breaks or whitespace in sensible places.

@alexcrichton are you aware of any s-expr pretty printing libraries we could use?

[alexcrichton]

Nah unfortunately not that I'm aware of :(

Crates like wasmprinter just hand-roll their own management of indentation levels and such.

[abrown]

As discussed in today's WASI meeting (cc: @sunfishcode, @lukewagner), if/once WebAssembly/interface-types#132 is merged then this PR should be replaced by some tooling to do some form of "WITX-> WIT" conversion (i.e. generating an official core-wasm signature for the high-level WITX).

[sunfishcode]

Since this PR was filed, a lot has changed. Using wasm-tools, the signature of a component can now be printed using wasm-tools component wit.