Avoid an unnecessary instantiation of do_numeric_format.
More thorough testing of formatting pointers.
Remove 'catch (const char *)' clauses from fmt.cc.

We no longer diagnose various syntax errors by throwing string constants,
so all the exceptions that should theoretically be possible inside
fmt.cc originate from the C++ runtime and _should_ all be subclasses of
std::exception.  The 'catch (...)' clauses are preserved Just To Be Sure.
We continue to special-case catch const char * in exceptions thrown by
user-definable conversions.