Float hexponent support: "%a" / "%A"

[charlesnicholson]

Does anyone actually use this? I don't get it.

converts floating-point number to the hexadecimal exponent notation.

For the a conversion style [-]0xh.hhhp±d is used.
For the A conversion style [-]0Xh.hhhP±d is used.
The first hexadecimal digit is not 0 if the argument is a normalized floating point value. 
If the value is ​0​, the exponent is also ​0​. Precision specifies the minimum number of digits 
to appear after the decimal point character. The default precision is sufficient for exact 
representation of the value. In the alternative implementation decimal point character is
 written even if no digits follow it. For infinity and not-a-number conversion style see notes.

[charlesnicholson]

Either way this should do exactly what %e does but just pass base 16 to npf__ftoa_rev or npf__etoa_rev or whatever. npf__ftoa_rev shouldn't take a base if npf__etoa_rev ends up being necessary.