Add a new low-level `describe` output for inspecting the canonical ABI details of

This is a potential alternative to #965 but they aren't necessarily mutually exclusive.

The goal here is to create an easy way to see the concrete canonical ABI details for a particular WIT, on the command line.

This new "generator" would output the size and alignment of types, as well as the offsets of their fields where appropriate.

For example, given

```
point {
x: u32,
y: u32,
}
```

it would create output something like

```
record point:
size: 8
align: 4
fields:
x: 0
y: 4
```

Basically a human-readable dump of the IR that `wit-bindgen` produces.

For functions, it should similarly produce a human-readable version of the operations required to lift/lower as well as the core Wasm signatures.

cc @titzer