Improve the .wasm glue code layer

[PabloMansanet]

At the moment the shims around the Player trait are a bit rough, and it could be a big win to automate as much of the process as possible (within reason) so we don't waste a lot of time keeping the glue code up to date manually as the Player<->World interaction changes.

Some pointers:

• This blog post series, even though it uses wasmer instead of wasmtime, remains the best reference I've found since he's trying to solve a problem almost identical to ours.
• Wasmtime supports a much better way of managing imports and exports to what's currently in the codebase, using a Linker. Doing this properly would also solve some annoying panics I found while testing (for example a Player module that doesn't actually look into the result of inspect at all would cause the import symbol not to be emitted, which would currently cause a panic as our spawn_player function expects exactly one import).
• It would be good to think early of some basic form of metering, to handle the case where someone uploads a loop hero (of the loop {} kind) and stalls the entire process.

[PabloMansanet]

I'm going to assign myself to this as I'm already in tha headspace, but I'll timebox it as I have the suspicion this is the kind of problem you can spend literally as many hours as you want on :smile:

[PabloMansanet]

Implemented in #16