Make external dependencies optional

[OrangeHoopla]

Made ab_glyph and rustdct optional and the respective modules they are being used in features. The only other dependency that doesn't span multiple modules is approx which would make geometric_transformations a feature if we wanted to make it optional.
note: this will break other people's use of this library if they have the default-features set to false and are using either drawing::text or image_hash

[cospectrum]

making DistanceFrom public isn't related to optional dependencies

[OrangeHoopla]

making DistanceFrom public isn't related to optional dependencies

My bad @cospectrum that was for another commit. I removed it.

[cospectrum]

There is 1 new image hash function on master which does not require dct and there will be more. So could you please replace image_hash feature with fft feature (not dct, since fft is the main shared dependency, this will help minimise future feature bloat). You will need to cfg feature on hash function instead of the entire module.

[OrangeHoopla]

I started working on this and it causes a cascade of warnings if/when the feature is removed. Would you rather I go through add #[cfg(feature = "fft")] to all imports that are not being used when the feature is removed(a little ugly imo) or do we want someone to see warning when they remove this feature?(probably not the best thing either)

[OrangeHoopla]

Nvm I think a mixture of #[allow(dead_code)] would be the solution.

[cospectrum]

I don't like unused imports. They still may slow down compile time I think. Lets put entire modules such that phash, signals, bits behind the feature

[cospectrum]

try to remove allow(dead_code) in this file

[cospectrum]

1 thread is remaining. You can do it!

[OrangeHoopla]

...sorry about that I got pulled away from the computer for a bit.