vlc_vector: add helpers for vectors

Add a new API for handling general-purpose vectors, intended to replace
ARRAY_*.

Like ARRAY_*, it provides macros to handle a dynamic array generic
over the type of its items.

Contrary to ARRAY_*:
 - it does not abort on allocation failure (but reports the error);
 - it uses size_t instead of int to store the capacity and the size;
 - it checks for overflows on reallocation.

For illustration purpose, embedding a vector of input_item_t* in a
struct looks like:

    struct playlist {
        struct VLC_VECTOR(input_item_t *) items;
        // ...
    };

The main features (with names inspired by std::vector from C++ and
std::vec::Vec from Rust) are:
 - void vlc_vector_init(pv)
      init the vector (use VLC_VECTOR_INITIALIZER for static init)
 - void vlc_vector_destroy(pv)
      destroy the vector and release any associated resources
 - void vlc_vector_clear(pv)
      remove all items from the vector
 - vec.size
      read the size of the vector
 - vec.data[i]
      access the i_th item
 - bool vlc_vector_push(pv, item)
      push an item to the end of the vector
 - bool vlc_vector_push_all(pv, items, count)
      push serveral items to the end of the vector
 - bool vlc_vector_insert(pv, index, item)
      insert an item at index
 - bool vlc_vector_insert_all(pv, index, items, count)
      insert several items at index
 - void vlc_vector_move(pv, index, target)
      move an item to target
 - void vlc_vector_move_all(pv, index, count, target)
      move a slice of items to target
 - void vlc_vector_remove(pv, index)
      remove an item
 - void vlc_vector_remove_slice(pv, index, count)
      remove a slice of items
 - void vlc_vector_swap_remove(pv, index)
      remove an item in O(1) without preserving ordering
 - bool vlc_vector_reserve(pv, mincap)
      increase the capacity of the vector in advance
 - void vlc_vector_shrink_to_fit(pv)
      resize the vector to its actual size
 - void vlc_vector_index_of(pv, index, pidx)
      find the index of an item (returns the result in *pidx)
 - vlc_vector_foreach(item, pv)
      a for-each loop

It uses an exponential growth policy for both increasing and decreasing
the size. As a consequence, the amortized complexity of
vlc_vector_push() is O(1).

Signed-off-by: Thomas Guillem <thomas@gllm.fr>