Account is now always a non-pointer. This will be important once
the manager starts remembering accounts.
AccountManager is now always a pointer because it contains locks
and locks cannot be copied.
* Change account signing API to two sign functions;
Sign without passphrase - works if account is unlocked
Sign with passphrase - always works and unlocks the account
* Account stays unlocked for X ms and is then automatically locked
* Use crypto.Sign instead of directly calling secp256k1 lib
* Rename UserAccount to Account and Addr to Address (for consistency)
* Change AccountManager.Sign to take ptr to Account instead of
address byte array
* Simplify copying of Accounts in Accounts()
* PubkeyToAddress and GetEntropyCSPRNG now exported
* Add initial UserAccount and AccountManager structs
* Add NewAccount, Sign and Accounts functions
* Refactor key stores to use key address as main identifier
while keeping the UUID.
* Use key address as file/dir names instead of UUID
