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
It's for E2E's S2K: https://github.com/coruus/e2e/blob/s2k-sha256/javascript/crypto/e2e/openpgp/s2k.js

Performance tests here: https://github.com/coruus/e2e/blob/s2k-sha256/javascript/crypto/e2e/openpgp/s2k_test.html

It's roughly 4x faster, because S2K only needs a few message schedule computations, but many hash updates. 

(And see E2E issue 113: https://code.google.com/p/end-to-end/issues/detail?id=113 )