Added missing braces
Moved function call to the top
Added __ws_nonnull__() calls
Exchanged nullchecks by __ws_nonnul__()
Fixed deinit call
Commented out some functions

They are not yet implemented in the ws_set class. So it does not make
sense to implement them in the value_set class.

Function names are:
ws_value_set_union
ws_value_set_intersection
ws_value_set_xor
ws_value_set_is_subset
Added error section to ws_value_set_new()
Removed explicit description of error codes
Added comment for struct declaration
Added link to errno.h
Fixed two comments
Added ws_value_set_select()