Merge #557: Eliminate scratch memory used when generating contexts

Merge #557: Eliminate scratch memory used when generating contexts

b3bf5f9 ecmult_impl: expand comment to explain how effective affine interacts with everything (Andrew Poelstra)
efa783f Store z-ratios in the 'x' coord they'll recover (Peter Dettman)
ffd3b34 add `secp256k1_ge_set_all_gej_var` test which deals with many infinite points (Andrew Poelstra)
84740ac ecmult_impl: save one fe_inv_var (Andrew Poelstra)
4704527 ecmult_impl: eliminate scratch memory used when generating context (Andrew Poelstra)
7f7a2ed ecmult_gen_impl: eliminate scratch memory used when generating context (Andrew Poelstra)

Pull request description:

  Builds on #553

Tree-SHA512: 6031a601a4a476c1d21fc8db219383e7930434d2f199543c61aca0118412322dd814a0109c385ff1f83d16897170dd0c25051697b0f88f15234b0059b661af41