Explicit support for angle type

Created by: stephanemagnenat

Issue #783 discusses the addition of boolean variable. For me this brings a more general discussion about types in the context of education. This issue aims at discussing the addition of an explicit angle type.

Currently angles are mapped to 16-bit integers taking advantage of the 2-complement implementation to provide wrap-around for free, but that is low level and not very nice for education. I would prefer explicit angles with constant literals, so that one can write var alpha = 30 degrees and var beta = 1.34 radian. Without implicit conversion, that would be a breaking change as the trigonometric functions would take angles instead of integers.