Add 64bit support

[Ughuuu]

I integrated rapier and rapier-f64. Then I added salva, but now it doesn't build and salva only works with rapier, not rapier64(eg. features="rapier")
https://github.com/appsinacup/godot-rapier-2d
appsinacup/godot-rapier-physics#61

One solution could be to add same as for rapier, single or double builds, and then for feature add rapier or rapier-f64.

[Ughuuu]

For this issue one should look at how rapier does it: https://github.com/dimforge/rapier
And basically it needs in salva multiple packages exported, similar to how rapier has. So right now this exports salva2d and salva3d, it would need salva2d-f64 and salva3d-f64. Each of these would be defined in the Cargo.toml file and then there would be a define for the precision:

• https://github.com/dimforge/rapier/blob/master/crates/rapier2d/Cargo.toml#L32 (Rapier defining f32 feature)
• https://github.com/dimforge/rapier/blob/master/crates/rapier3d-f64/Cargo.toml#L32 (Rapier defining f64 feature)

So in the build directory of salva there would be 2 new folders, salva2d-f64 and salva3d-f64. Then in github workflows there needs to be automatic building (and deploying?) of the new packages.

Example of how to use the f32 and f64 flags:

• https://github.com/dimforge/rapier/blob/master/src/lib.rs#L19