It now exposes a type alias for Word64 and relies on the user to supply
a value for the current time, allowing them to use the time library of
their choice.
Also bump memory dep to 0.12 and use fromW64BE from that library.
While HOTP only mentions SHA1, TOTP allows the use of different hash
functions, which implicitly requires that the HOTP implementation support
them too.
This will also allow users to use HOTP with another hash if they so choose,
though it would not be compatible with most client applications, such as
Google authenticator.
Reduce the arguments to the totp function (most people will use defaults)
and allows validation of the time step value.
Added a top-level module overview.
