For consistency, support binary and octal floating-point, and accept
a "0d" or "0t" prefix for decimal floating-point. However, we do not
accept a binary exponent (p) for a decimal mantissa, or vice versa.
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.
