Hash functions for Bitcoin

extends hashlib micropython module with ripemd160 and sha512 functions.

Also adds a single-line function for pbkdf2_hmac_sha512:

pbkdf2_hmac_sha512(password, salt, iterations, bytes_to_read)

in Bitcoin to generate a seed:

pbkdf2_hmac_sha512(mnemonic, 'mnemonic'+password, 2048, 64)

TODO:

• make API the same as in normal python
• make C-optimized hmac and pbkdf2 versions with standard python API