Module linking

[chrisdickinson]

While we allow multiple Wasm modules to be passed in via the manifest, we don't currently link them together like the main Extism library does. This is related to #9, in that this requires adding a step between ArrayBuffer and WebAssembly.Instance so that we can examine imports and exports and create appropriate trampolines.

[chrisdickinson]

I started taking a look at this this morning to shake off the rust from Thanksgiving break. Here's a grab bag of observations:

1. I had initially thought that we could late-bind using trampoline objects - that is, collect all of the modules by name before instantiation, collect all of their exports, and build a single import map that implements all module names and functions by delegating calls between the module instances – basically late-binding everything. Of course, I had forgotten about shared memories, tables, and values – which means we can't fully get away with that approach! We'll ultimately need to do a little topographical sorting to get it to work.
2. name becomes more important under this model. We'll want to align with how the Rust extism runtime infers name when unavailable [1].
3. Wasmtime's Linker uses different instantiation strategies for "command" and "reactor" modules, we'll want to do something similar [2]

I'll see about pushing this forward over the next couple of weeks, but it's not likely to make the 1.0 beta deadline.