Less dependencies

[nilgoyette]

Remove num and some default features. Not much but a cargo build --release --features ... was 2.5s faster on my work computer.

I tried getting rid of nalgebra because it's huge, I'm not using much of it (3x3 and 4x4 matrix, inverse, eigen and SVD) and we need to update it from time to time ... but it seems like an impossible job. The best I could find was default-features = false and it's not doing much!

I'll send a nalgebra update PR soon (0.18 to 0.21), if it's ok with you. EDIT: we might as well update all dependencies...

[Enet4]

These changes are OK. 👍 Thanks!

I'm keeping a note that this may be perceived as a breaking change, since a dependency in the public API has been stripped of its default features.