e2e/openpgp/s2k.js: Fixed the long-password-length case in getKeySha256_. Integrated dispatch to getKeySha256_ into getKey.

e2e/openpgp/s2k_test.html: Added performance test to compare SHA2-256 without prescheduling, SHA2-256 with prescheduling, and SHA1 for deriving a 32-byte key.