JS bindings changes

[kddnewton]

• Overall the biggest change is the main entrypoint now accepts a Uint8Array instead of a string in order to accomadate actually properly parsing without going through the UTF-16 JS intermediary.
• Added a Location and Source class to the JS bindings to make them a little more ergonomic. Also using them in the playground.
• Learned a ton about TextEncoder and TextDecoder, started properly using these APIs.
• Cut down quite a bit on the WASM size, because now we're only exporting the 7 functions we actually need, as opposed to exposing the entire footprint.