Added support for num_traits::identities.

[virtualritz]

This is for #157.

• The impls are in src/vec/num_traits.rs.
• This is behind a resp. feature flag documented in src/lib.rs and README.md.
• I took the liberty to reorder the features alphabetically (except f64 & int as these are probably what people look for most).
• clippy clean.

I noticed three serde tests are failing when cargo test --all-features is ran but they did so before my changes:

test impl_serde::mat_serde_tests::mat2 ... FAILED
test impl_serde::mat_serde_tests::mat3 ... FAILED
test impl_serde::mat_serde_tests::mat4 ... FAILED

[virtualritz]

I forgot, I didn't add tests as these additions so far are just wrappers around exisiting functions.

[fu5ha]

Great, thanks! Only thing is could you add a note to the changelog? :D

[virtualritz]

Done!