Cache trig and combine deg2rad in heading()

[gistrec]

Optimize heading() hot-path

Two micro-optimizations in geo::heading() — it's called in every bearing calculation, so each saved instruction adds up.

Combine two deg2rad calls into one
Instead of converting from.lng and to.lng separately and subtracting, compute deg2rad(to.lng - from.lng). Algebraically identical (a*k - b*k == (a - b)*k), one fewer multiplication, and the from-side longitude is no longer needed in radians at all.

Cache sin / cos of latitudes
cos(to_lat), sin(to_lat), cos(from_lat), sin(from_lat) each appeared twice in the atan2 arguments. Without -ffast-math, the compiler is not allowed to CSE calls to std::sin / std::cos because they may touch the floating-point environment (errno, FE flags). Storing them in locals forces a single libm call per value — cuts trig calls in this function from 8 to 4.

Behavior is unchanged — codegen only