* Decrease bucket size

A bucket size of 8192 bytes is quite large and it turned
out that this can exhaust the available heap space too
too quickly.

This is because even for allocating 1 byte a bucket of
8192 bytes is allocated/wasted.

* Return 0 if requested size too large

* Improve test

The test didn't use an offset when setting up the heap.
Hence the first successfully allocated pointer was
always `0`.

This is unfortunate since `0` is also the return value
when there is an error.

This lead to us not noticing that the test was failing,
because it did not distinguish between success and error.

* Revert to linear allocator