feat: initial codegen

[tjjfvi]

Makes progress towards #226

This cleans up the codegen code somewhat, though it's still not the prettiest. It also adds codegen for codecs, as well as initial stub codegen for pallets and their storage and calls.

This has only been minimally manually tested; automatic testing should be added, but we need to figure out the best approach for this.

[harrysolovay]

Tested with Polkadot and the output looks correct.

This is a great foundation: types, corresponding codecs (each explicitly typed to $Codec<T> and named in accordance with their idx, specialized codecs drawn into scope, tsdocs wherever possible, and even the beginnings of extrinsic / storage bindings! Awesome work @tjjfvi.

Would love to be able to use it through something like...

deno run -A -r https://deno.land/x/capi/codegen.ts \
  --src="wss://rpc.polkadot.io" \
  --out="polkadot.ts"

[harrysolovay]

Looking very nice. Review is mostly nits––pretty much merge-ready.

[harrysolovay]

Awesome work @tjjfvi