sscanf prim (SF issue)

[tanabi]

with the fmtstring prim we Fuzzball now has something like C's
sprintf. It would be nice if we had something like sscanf as well
which would do:

sscanf[ str:text str:format -- ?1 ...?n n ]

It would take a standard C scanf pattern and match the string. If
the string didn't match, n would be 0.