Angle<> needs alternative representation control

[FrancoisChabot]

Right now, Angle<>'s internal representation is fixed to be ]-pi.pi]. But that's not great because:

• It only works with floating point storage
• There is a fair bit of wasted bits.

I could easily see something like:

using Angle = vecpp::Angle<Fixed_point_representation<std::int16_t>;

Coming in handy.

However, while this is a nice to have, it's a LOT of work that addresses no particular imediate need.

One thing we could seriously consider is modifying the user-facing API immediately to reflect this so that the eventual introduction of alternate angle internal representations does not induce a breaking change.