Removes the dependency of jsrsasign and replaces it with
https://developer.mozilla.org/en-US/docs/Web/API/Crypto/subtle
Which has reasonable browser support and a shim otherwise.
https://caniuse.com/#search=SubtleCrypto
https://github.com/vibornoff/webcrypto-shim

This change introduces two breaking changes

• If you are using a browser unsupported by SubtleCrypto then you'll need the pollyfill
• If you implement ValidationHandler then you'll need to update validateAtHash to return a promise

You should be saving up to 227kb off your bundle size with this chance since jsrsasign is completely removed. See #196
This also makes this library more compatible with downstream typescript compilers, such as when being used with Bazel's ts_library