Eliminate scratch memory used when generating contexts

Summary:

• ecmult_gen_impl: eliminate scratch memory used when generating context
  • ecmult_impl: eliminate scratch memory used when generating context
  • ecmult_impl: save one fe_inv_var
  • add secp256k1_ge_set_all_gej_var test which deals with many infinite points
  • Store z-ratios in the 'x' coord they'll recover
  • ecmult_impl: expand comment to explain how effective affine interacts with everything

This is a backport of secp256k1's PR557

Test Plan:

ninja check-secp256k1

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4972