vinyl: refactor index creation and deletion functions

We can't move vy_index_new() to a separate file right now, because it
depends on vy_env and we can't just replace vy_env with vy_index_env and
vy_cache_env there, because the function is called from outside vinyl.c.
To deal with that, let's introduce wrappers around vy_index_new() and
vy_index_delete(), vy_new_index() and vy_delete_index(), which are vinyl
C API functions that simply call the internal functions with appropriate
arguments. Also, remove vy_index_ref() and vy_index_unref() from the C
API, as they are not really necessary.

Needed for #1906