A wasi_unstable description based on module types.

This sketches out a description of the current wasi_unstable API, using a syntax which anticipates the "module types" syntax [recently proposed to the CG](https://github.com/WebAssembly/meetings/blob/master/2019/CG-08-06.md#discuss-new-proposal-that-introduces-types-for-modules-and-instances-and-text-format-thereof-as-initially-discussed-in-design1289), though it does use a few extensions which can be easily lowered, and which mostly anticipate the [Interface Types](WebAssembly/interface-types#47 (comment)) proposal.

This is derived from [the WatIDL proposal](WebAssembly#64), though it differs in a few ways:
 - It doesn't yet do anything special for capabilities, for now. File descriptors are just integers, for now. But the idea is that we could add OCAP concepts later.

 - It uses a new `(flags ...)` construct in place of structs of Bool fields, to describe flags values. This allows us to explicitly declare the underlying wasm type for flags values.

 - Types used by the API are declared outside of the moduletype, because we're using wat syntax as much as possible, and there, `(type ...)` inside a module declares an entry in the type section, which isn't what's intended here.

The extensions to module types are:
 - It uses `string`, `array`, and `{s,u}{8,16,32,64}` types, which are expected to be provided in some form by the Interface Types proposal.  For now though, they can all be lowered in straightforward ways, by assuming that `string` is always just UTF-8.

 - It adds `typename` declarations, which can name any wasm type, and of the extra types mentioned in the previous item, or `struct`, `enum`, or `flags`.

 - It declares functions with multiple return values, which for now will need to be lowered to output (pointer) parameters.

Author: sunfishcode