Generate Python code via a WASM plugin

[kyleconroy]

Based on #1406 and https://github.com/kyleconroy/go-wasm-plugins, this is a working, proof-of-concept implementation of generating code via a WASM plugin. It generates the same Python code, but over 10x slower!

$ time WASMTIME_BACKTRACE_DETAILS=1 sqlc-dev generate --experimental

real	0m0.035s
user	0m0.028s
sys	0m0.011s
Kyles-MacBook-Air:python kyle$ time WASM=1 WASMTIME_BACKTRACE_DETAILS=1 sqlc-dev generate --experimental

real	0m0.955s
user	0m0.932s
sys	0m0.027s

EDIT: After reading over the docs, I implemented module caching be serializing the module after initializing it from the WASM. This significantly sped up module initialization, to the point where the WASM version is barely slower.

time WASM=1 WASMTIME_BACKTRACE_DETAILS=1 sqlc-dev generate --experimental

real	0m0.055s
user	0m0.044s
sys	0m0.017s