Proof of concept: angle types

[klukasiuk]

Add strongly typed angle units (Degrees and Radians) with conversions and arithmetic

[sbarral]

It would be good to add a design rationale, in particular why we would want to design our own when crates like uom, angular_units and ang already exist.

uom in particular covers much more ground and I prefer its design in the sense that type parametricity only concerns the quantities (Length vs Angle vs Mass etc) but not the units themselves. An angle in degree and an angle in radians are the same type in uom, the unit is only seen at the boundaries when defining constants and reading a result. In this design and in angular_units (I haven't looked at ang), the way the type of the sum of degree and radian angles is chosen feels arbitrary.

Defining conversion functions like Degree::to_radians may not scale too well if we want to generalize to quantities like length etc which have many common units.