Set up build with wasm-bindgen and rollup to inline wasm

[steos]

This removes use of wasm-pack and instead builds directly with cargo and wasm-bindgen and adds a rollup step to bundle the result with wasm inlined.

Where the build process previously was simply to run wasm-pack, write a package json and copy files (license, readme, typedefs) we now need to

• build with cargo and generate bindings with wasm-bindgen the intermediate output of wasm-bindgen is written to build/wbg_ristretto and build/wbg_p256
• generate an entry module wrapper for rollup and write to each intermediate build output
• run rollup for both builds (wbg_ristretto and wbg_p256)
• write the package.json for each package (build/ristretto and build/p256)
• copy license/readme/typedefs
• amend typedefs with ready export

all of this happens now in bin/build.js