This commit switches wasm-pack to using a global cache for all download
binaries, living typically in a user's home directory. The intention
here is to aovid creating a `bin` folder in all wasm-pack projects and
additionally share downloads between projects to ensure that you're
downloading a minimal number of binaries from the network.
Along the way the downloading code was restructured to support a global
cache, but everything should largely be as it was before!
Closes#292
This lets us leverage `cargo` for semver finding and then ensure that we get the
exact same version of the CLI that cargo selected. It also lets us support fuzzy
dependencies like "0.2" instead of exact dependencies like "0.2.21" again.