Revise the whole terminology one more time

[cher-nov]

I still don't like e.g. GENA_USE_SAMPLE, GENA_USE_ENTITY and so on, as they're not intuitive.
The new approach to defining a terminology should be next:

• avoid homonyms;
• prepare an initial list of words that can be used, then select from them by considering intuitivity;
• ... (I'll think about it later, now I'm slightly discouraged).

[cher-nov]

Style fix required: first brace must be at the same line.
https://github.com/cher-nov/Gena/blob/b9cf7ac50898a2298ab0eb1db7dd8c6723f055c5/genvector/genvector.c#L17-L18