Ticket 457: Generalize ASCII digits to all Unicode decimal digits

Currently, only ASCII digits are supported by string->number, read, and Scheme numeric literals. However, the numeric-char? and new digit-value procedures return #t and the numeric value on any Unicode decimal digit that the Scheme implementation provides. (Decimal digits have a general classification code of Nd.) Unicode provides 46 sets of decimal digits, all allocated to sub-blocks of 10 consecutive codepoints.

This proposal requires string->number, read, and Scheme numeric literals to support all Unicode decimal digits that are supported by the application.