#53 string to number conversion

[Mark Lentczner]

string.as-number and string.is-number are currently limited to only
decimal integers within the 32-bit signed int range. They should
support a wider variety of numbers including floating point.

Further, they should have optional arguments that support other
bases, and base-prefixes.

Questions? Trim white-space option? Ignore trailing non-digits?
Support prefix chars like '$' and '+'? Ignore embedded spaces?
Support embedded comma?