std.math.complex: add imaginary unit, other breaking changes, etc.

[youdie323323]

New imaginary unit

Previously, users had to manually define the imaginary unit when working with complex numbers, like this: const i: Complex(T) = .init(0, 1);.

For instance, what used to look like:

const i: Complex(f32) = .init(0, 1);

...

const i_diff_2 = i.mul(diff_2);

Can now be written more cleanly as:

const i_diff_2 = diff_2.mul(.i);

or:

const i_diff_2: Complex(f32) = .mul(.i, diff_2);

(don't forget that you can actually use mulByI).

This PR includes addition of a built-in imaginary unit constant to Complex(T), which simplifies things a bit.

Others

Changes

• Complex(T).mulByMinusI is added -- this will multiplies -i to a complex number.

Breaking Changes

These renames are for consistency with other naming conventions:

• Complex(T).conjugate has been renamed to Complex(T).conj.
• Complex(T).mulbyi has been renamed to Complex(T).mulByI.
• Complex(T).reciprocal has been renamed to Complex(T).recip.

Other breaking changes:

• std.math.complex.conj has been removed. Use Complex(T).conj instead.

[youdie323323]

Since there's already conjugate method implemented in Complex, should we just remove math/complex/conj.zig? (and maybe rename conjugate to conj, as math/complex/conj.zig did it?)