Add a function for extracting digits, sign, and base prefix from a string but not calculating the final number

[jwodder]

The function should take a &str starting with a number and, on success, return a structure containing:

• the Base of the number as determined from the prefix (if any)
• an enum for the sign of the number
• the digits of the number as a Vec<u32>
• the remainder of the &str

This could be used to give strtoint-like functionality to libraries that provide custom integer types without having to make StrToInt generalizable to & implementable by all possible integer types.

Add variants of this function based on/corresponding to #3 and #4.