Create witx-type-representation.md

[monoclex]

This document summarizes the types that can be found in the phaser snapshots. The goal of this is that it should be a one-stop shop for people looking to implement ABI compatibility with certain types. This PR was created to resolve one of the issues brought up with #316

Points of improvement before merging:

• More condensed representation of whole bytes? subscription_u would be hard to read on mobile
• Pointer needs more elaboration
• Are the images exactly accurate to the positions of bytes?
• Will it answer all ABI protocol related questions? Are there odd edge cases?

[sunfishcode]

Cool! This looks like a great start documenting a part of witx which is currently undocumented!

Could you add a link to this from here? https://github.com/WebAssembly/WASI/blob/master/docs/witx.md

One other thing which may be useful to mention in the intro; the representations of these types are exposed when used with linear memory, however witx is also meant to be used in the future in non-linear-memory contexts as well, in which case the representation is opaque.

[sbc100]

Nice!

I'm sure folks will find this useful.

I wonder if it makes sense to integrate this into the existing docs somewhere? Or at least be linked to from witx-generated docs? @pchickey

[monoclex]

my bad on taking forever lol

[lygstate]

The offset may be 4 or 8 depends on sizeof pointer

[monoclex]

It seems that the docs don't mention this distinction - I'm unsure if it's within scope of this PR to mention it if the docs don't.

[monoclex]

Apologies for it having been a while since I've worked on this. I tried to update the terminology to be more in-line with what's currently in use, but I might've glossed something over.