Added semantic versioning.

Fixed warnings, general code style cleanup.

Removed ability to define comparison callback at compile-time.
`skiplist_new` always takes a comparison function pointer,
which is required.

Memory allocation callback (and a user data `void *`) are now
arguments to `skiplist_new`. Both are optional, and can be NULL.

Changed the iteration API to use a CONTINUE/HALT enum return value,
since a real return value of a user-chosen type can still go in the
environment passed to the callback.

`first, last, pop_first, pop_last`, and others now return `bool`
rather than an int (which was used like a bool).

Use more appropriate numeric types: `size_t` rather than
`long` for `skiplist_count`'s return value, for example.

See CHANGELOG.md for other details.