Remove CharToBool

This utility function isn't really necessary since it's easier to say `c != 0`
than it is to say `CharToBool(c)`.
Don't export utility functions

These don't have anything to do with Rocksdb and really shouldn't be a part of
the public interface.
Remove gorocksdb_get_{char,int}_at_index in favor of Go functions

Go doesn't have pointer arithmetic, but it has slices, so we can use those
instead of making expensive C calls.
Merge pull request #6 from edsrzf/utility-cleanup

Utility function cleanup