Add the exponential notation for floats

Fixes #6593

This change the `std::num::strconv::float_to_str_common` function to optionally format floating point numbers using the exponential (scientific) notation. The base of the significant can be varied between 2 and 25, while the base of the exponent can be 2 or 10.

Additionally this adds two new formatting specifiers to `format!` and friends: `'e'` and `'E'` which switch between outputs like `1.0e5` and `1.0E5`. Mostly parroting C stdlib in this sense, although I wasn't going for an exact output match.