Add scientific notation format specifier for floating point values

[SiegeLord]

This would constitute an addition of two new formatting flags, {:e} and {:E} and corresponding traits (e.g LowerSci and UpperSci). Here's the output I envision based on how Rust handles floating point and signs today. Notably, this differs from how C does it (Rust already differs from C, see #1375):

format!("{:e}", 1.0) == "1e0";
format!("{:e}", 10.0) == "1e1";
format!("{:+e}", 10.0) == "+1e+1";
format!("{:e}", 1.1234567) == "1.123457e0";
format!("{:e}", 1.3e-1) == "1.3e-1";
format!("{:.2e}", 1.3e-1) == "1.30e-1";
format!("{:E}", 1.3e-1) == "1.3E-1";

[huonw]

Duplicate of #6593. (FWIW, I'm personally not keen doing this with new format specifiers, when we can just use newtype structs; others may differ in this opinion.)