Crypto related cleanups.

Moved Bunch of functions from net_crypto to crypto_core.

decrypt_data_fast and decrypt_data_symmetric were the same thing
therefore, removed decrypt_data_fast.

Replaced all the crypto_secretbox_* defines with the equivalent
crypto_box_* one.

New define: crypto_box_KEYBYTES that is equal to
crypto_box_BEFORENMBYTES.
Fixed a test.

Removed function from Messenger.h