Faster SE(3) and SE_2(3) exponential maps

[dellaert]

DexpFunctor was instantiated several times:

• for computing R via SO3::Expmap
• for ExmapTranslation, twice so in NavState

This PR

• gets rid of ExpmapTranslation and inlines it in the 4 places it was used;
• in the Pose3 and Navstate Expmaps, we also now use the one instantiated DexpFunctor to compute the rotation;
• according to Barfoot Eq 8.83, X = J_r * inv(J_l) = R^T, so substitute that wherever we used X before: another big saving!

This PR saves a bunch of compute (many sines and cosines, and computation of skew W and W*W) and is more "transparent" about what is going on. In addition, the X=R^T saves even more on top of that.

[dellaert]

@timbarfoot I was inspired to look at this code more critically because of you :-)