A general-purpose queue data structure

We're hoping to address the main problems that were exhibited by the stack and list data structures:

• We should be able to manage the data without knowing what it is.
• We'd like operations, such as appending, to be independent of the number of items already stored.
  Such (highly desirable) operations are performed in a constant-time.

typedef struct _e {
    void        *data;
    size_t      datalen;
    struct _e   *next;
} ELEMENT;

typedef struct {
    ELEMENT     *head;
    ELEMENT     *tail;
} QUEUE;

Of note:

• We've introduced a new datatype, ELEMENT, to hold each individual item of data.
• Because we don't require our functions to "understand" the data they're queueing, each element will just hold a void pointer to the data it's holding, and remember its length.
• Our "traditional" datatype QUEUE now holds 2 pointers - one to the head of the list of items, one to the tail.