adding/implementing BlockCipherMode.array_concat(), an efficient array concat method.

[orthecreedence]

Tested that MD5s from encrypting larger files matched before/after these changes (they do).

before changes (336kb file):

FF:      11,506 ms
Chrome:  21,190 ms

after changes:

FF:         809 ms
Chrome:     476 ms

The main bottleneck is now in Firefox in aes.js (line 133) block_encrypt. I tried some performance tweaks there but nothing worked out so I reverted everything. Firebug's profiler is generally good about finding the sub-functions that are taking the most time, and in this case it says that mix_columns is the slowest, HOWEVER mix_columns is reported to only take up about 1/4 of the time that block_encrypt uses. I don't see any realy math in block_encrypt, just a loop and some calls to other functions, so I don't know where that number is coming from.

Anyway, chrome now encrypts at about a rate of 1mb/s, and firefox at about 0.5mb/s.

[orthecreedence]

OH please note that although I updated all the supported block ciphers with the new array_concat method, I only tested CBC. It should work but please test before merging!