Build CryptoJS components into webcrypto.js

We only depend on cryptojs for this webcrypto polyfill, so let Grunt
concatenate them into one file.

The reference in the getString helper isn't needed since we use the
built in string converters on CryptoJS's word arrays.
Simplify webcrypto type conversion

Previously we'd get a WordArray and convert to string before converting
to array buffer. Instead, go directly to array buffer.
Do fewer string conversions in tests