Inline Jet operators and implement compound assignments in-place

- Implement Jet compound assignment operators (+=, -=, *=, /=) in-place
  instead of constructing temporary Jets (*this = *this + y). Scalar +=
  and -= now only update the scalar part .a without touching .v.
- Initialize .a via the member initializer list in Jet constructors.
- Add EIGEN_STRONG_INLINE across Jet constructors, compound assignments,
  arithmetic operators, and mathematical functions.

Benchmark (Apple M1 Pro, kAutoDiff, kResidualsAndJacobians):
----------------------------------------------------------------------
Benchmark                                     Before    After   Change
----------------------------------------------------------------------
SnavelyReprojectionError                     78.4 ns  73.2 ns    -6.6%
ColmapFullOpenCVReprojectionError            86.2 ns  78.5 ns    -8.9%
ColmapSampsonError                           86.2 ns  79.6 ns    -7.7%
PoseGraph3dError                              139 ns   132 ns    -5.0%
----------------------------------------------------------------------

Change-Id: I184ceebad76cbb0eddc42be6368872a9e5543b6f