This introduces internal types:
* CKeyID: reference (hash160) of a key
* CScriptID: reference (hash160) of a script
* CTxDestination: a boost::variant of the former two
CBitcoinAddress is retrofitted to be a Base58 encoding of a
CTxDestination. This allows all internal code to only use the
internal types, and only have RPC and GUI depend on the base58 code.
Furthermore, the header dependencies are a lot saner now. base58.h is
at the top (right below rpc and gui) instead of at the bottom. For the
rest: wallet -> script -> keystore -> key. Only keystore still requires
a forward declaration of CScript. Solving that would require splitting
script into two layers.
so it takes a flag for how to interpret OP_EVAL.
Also increased IsStandard size of scriptSigs to 500 bytes, so
a 3-of-3 multisig transaction IsStandard.
OP_EVAL is a new opcode that evaluates an item on the stack as a script.
It enables a new type of bitcoin address that needs an arbitrarily
complex script to redeem.
