Make a proxy crate for the WASM REPL

[yannham]

As mentioned in a discussion, the instructions to manually build (understand without Nix) the WebAssembly REPL are complex and error-prone. Even with Nix, while usage is easy, the manual process is just incorporated as a build script, and thus it's not free either in maintenance or onboarding new developers.

The situation is due in part to current limitations of Cargo, but one technique that would make things slightly better and seems to be used elsewhere is to make a separate crate which just exposes the nickel crate but with the right setting for being built by wasm-pack. See the full discussion for details. Let's try that.