Convert vector code from cgmath to euclid.

[kpreid]

Gains:

• Potential no_std compatibility.
• Direct bytemuck and arbitrary implementations for vectors.
• Unit-of-measure system to distinguish coordinate spaces without newtypes.
• We may be able to get rid of GridMatrix because euclid::Transform3D doesn't mind integers. (I haven't tried that yet.)
• A more actively maintained dependency.

Losses:

• The vector types have an extra _unit field making pattern matching awkward.
• No angle-in-degrees type and operations.
• No Vector4.
• No suitable perspective projection function.
• No map() and zip() methods, so we need an extension trait (but on average fewer traits needed).
• Unit-of-measure system might be more trouble than it's worth.

There are a number of loose ends from this migration, which have been marked with the keyword "TODO(euclid migration)".