Version 0.3.0: support for both hex and decimal string construction

as requested by @jkryl.  Decided to go with a stringstream-based
implementation, as this is simpler by far than hand-rolled iteration
over *utf8.

(If someone wants to benchmark the two approaches, go ahead - but we use
stringstream elsewhere in Int64 already, so it seems uncontroversial to
add it here.)
Adding a couple of tests for decimal string constructors