* Fix running dist builds on tags
* Only run dist builds on tags, no need to run the full matrix.
* Fix dist builds on AppVeyor to include `wasm-bindgen-test-runner.exe`
* Only cache `~/.cargo` on Travis for the guide build, the `target` dir changes
too much to cache it.
This commit is an implementation of mapping u64/i64 to `BigInt` in JS through
the unstable BigInt APIs. The BigInt type will ship soon in Chrome and so this
commit builds out the necessary support for wasm-bindgen to use it!
