perf: optimize pubkey generation by removing unnecessary constant-tim…

[peiqing1998]

Hey folks.

Recently, while doing high frequency trading, I realized that the public key generation part could be faster.

I took a closer look at the code and found that some constant time operations in pubkey_create_helper can be optimized.

The change is very simple, that is, some of the operations that do not require constant time are replaced with variable time operations.

For example, scalar_set_b32_seckey is replaced by scalar_set_b32, ge_set_gej is replaced by ge_set_gej_var, and the use of constant time operations in these places is a bit of a killjoy.

The measured performance improvement is around 5-10%.

For high-frequency trading, a little bit faster is a little bit faster.

I have carefully reviewed the security aspects, the confidentiality is still confidential, just the fastest place to make it faster.
Benchmark numbers.

Before: ~123k pubkey/sec
After: ~132k pubkey/sec
Feel free to review and discuss any questions you may have.

Cheers!

[apoelstra]

As I mentioned in #792 -- you probably want to PR this upstream rather than here (in fact, our CI will reject these changes which make nontrivial changes to the vendored C code). I also don't believe that these changes are safe in general. But to consolidate discussion probably we want to discuss it on an upstream PR.