* renamed `_mulle_concurrent_hashmap_lookup_any` to `mulle_concurrent_hashmap_lookup_any` since its safe to pass NULL.
* renamed `_mulle_concurrent_hashmap_get_count` to `mulle_concurrent_hashmap_count`,
since it's safe to pass NULL and it's not a get operation.'
* improved the documentation
* added some more "safe API" routines for release
* improved the headers for readability
* clarified return codes of `mulle_concurrent_hashmap_remove`.
* changed internal representation of mask from unsigned int to uintptr_t,
because it's easier to read by the debugger
* does not use `errno` directly anymore, but instead returns the errno codes
as the return value (sometimes as negative numbers)
* change init error code to EINVAL, because that's what the other code uses.
* fix some gcc compile problems
* Adapt to changes in `mulle_allocator` and `mulle_aba`
* Remove dependency on `mulle_aba` for the pure library.
* Rename _free to _done.
* Merciful Release