Perf improvements for sha1.js, sha2.js, and sha2_64b.js

allocating additional memory (and incurring additional garbage
collections) unnecessarily. ~10% speed gain on Chrome 35.

goog/crypt/sha2.js: Port the block-size fast path from Sha1.update
to Sha2.update. Compute message schedule on the fly. Remove code
for testing for invalid inputs that is not present in Sha1.update.
~30% speed gain on Chrome 35.

goog/crypt/sha256_test.html: Remove testBadInput(); no similar test
is performed in sha1_test.html
Add a bit more to the comment to make the W optimization a bit clearer.  This optimization confused me more than it should have.  Maybe something like: "Traditionally W is precomputed as an 80-byte array.  However, only 16 elements are needed for any iteration so compute W on the fly and keep only the last 16 values."