Optimize gas usage in ecpair precompile

[birchmd]

Currently the ecpair precompile cannot be executed because it uses up more than 200 Tgas by itself. The purpose of this issue is to get this number down so that it becomes usable.

See #41 for information about measuring gas usage in pre-compiles.

[joshuajbouw]

Considering that the backend is widely used and regarded, and when I went through it was quite straight forward, I worry that this may be an issue until Math API is pulled in near/nearcore#3954.

[birchmd]

The ecpair precompile was not included in any performance improvement at the nearcore level. It still does not work because it consumes too much NEAR gas. The test_ecpair function in our standard_precompiles contract currently takes almost 2000 Tgas (that's not a typo, I really mean two-thousand teragas!).