Make set of valid strings for from_str_radix() consistent with primitives

[OTheDev]

In this PR, we change what constitutes a valid string slice in from_str_radix(), assign_str_radix(), from_str_base(), and assign_str_base() functions so that it is consistent with the string slice expected by from_str_radix() for primitive integer types (e.g. i32::from_str_radix()).

This is a BREAKING change, as previously, among other things, leading and trailing whitespace was permitted.

Moreover, assign_str_radix() and assign_str_base() functions no longer maintain the guarantee that the original value will remain intact if a parsing error occurs. In particular, these functions no longer validate the entire string before assigning to the arbitrary precision integer's underlying buffer. A warning has been issued in their respective documentation. This is also a BREAKING change.

Finally, we make from/assign string code much more maintainable by basing all functions on a single implementation: assign_str_base().

There is no performance degradation.