add `MP_SMALL_STACK_SIZE` option

add `MP_SMALL_STACK_SIZE` option

This adds an option to use a heap-buffer for the usually stack-based
`MP_WARRAY`-sized temporary buffers.

Per default it will reserve a single buffer, which can be modified
* at compile-time via the `MP_WARRAY_NUM` define
* at run-time by calling `mp_warray_init()`

The internal structure can only be created once. If one wants to modify
the maximum number of elements, the entire structure has to be free'd
by calling `mp_warray_free()`.

In case one wants to use this option with multiple threads, one shall
use the `mp_warray_init()` function and pass appropriate locking functions.

Signed-off-by: Steffen Jaeckel <s@jaeckel.eu>