crypto.ecdsa: improves internal sign_digest routine

[blackshirt]

This PR improves sign_digest internal routine used by PrivateKey.sign method. Its done by eleminating one step of C.EVP_PKEY_sign calls with null signature buffer to know the length (size) of required buffer. This maximal sizes of the buffer was already known by calling C.EVP_PKEY_size, so its not needed to call C.EVP_PKEY_sign just to know the size of required buffer and we can allocate the buffer instead and call C.EVP_PKEY_sign directly with this allocated buffer.

In my benchmark test, there are small improvements on this part

Benchmarking PrivateKey.sign() (before patch)...
Average PrivateKey.sign() (before patch) time: 37 µs

and after patch

Benchmarking PrivateKey.sign() (after patch)...
Average PrivateKey.sign() (after patch) time: 27 µs

Thanks

[huly-for-github]

Connected to Huly®: V_0.6-22355

[spytheman]

Excellent work.