Change "lookup" to "find"

The "find" verb seems to be more common for such kind of functions,
besides that it's already used in "sampgdk_find_symbol" and I'm going
to add "sampgdk_find_native" to the list in following commit.
Add sampgdk_find_native()

Now that we have sampgdk_call_native() and sampgdk_invoke_native() I can't
see any reason not to add sampgdk_find_native() as users will need to
implement a similar function anyway and we already have sampgdk_native_fid()
in internal code.

sampgdk_find_native() uses binary search (as natives are sorted by name),
so it's pretty fast. This might be another reason for adding it as most
people (I think) wouldn't bother implementing a binary search.