Optimizations derived from armfazh/flor-sidh-x64 project.

- optimizations.md file removed.
 - DblAdd function now is a method of ProjectivePoint and ProjectivePrimeFieldPoint.
 - A better (re)utilization of variables inside of DblAdd method.
 - Reflect package is not required anymore in p751toolbox/print.go.

Look at optimizations.md file.

Use "shared secret" instead of "key agreement" to point out that there's no hashing, key derivation, etc. going on.

Measure the keypair generation (including random sampling) in the "fast" benchmarks.

A later revision of the CLN paper uses the Montgomery relation to recover the a
value from the three x-coordinates in the public key, saving its transmission.
This patch improves slightly over the description in the paper, by
projectivizing the computation of the first four-isogeny; this means that it's
not necessary to perform an inversion to get the affine a value.

Currently these aren't tested; Sage gets unhappy working with isogenies of
large elliptic curves.  (Also, the previous test vectors were generated just to
test curve arithmetic, so they're not in the correct isogeny class and should
be changed...)