Allow any radix letter from the set [bydtoqhx] to be used either
"Intel-style" (0...x) or "C-style" (0x...). In Intel style, the
leading 0 remains optional as long as the first digit is in the range
0-9.
As a consequence, allow the prefix "0h" for hexadecimal floating
point.
Since we allow the prefix $ instead of 0x for integer constants, do
the same for floating point. No suffix support at this time; we may
want to consider if that would be appropriate.
