mem: various arena fixes

[tspader]

The allocator interface now requires a size for realloc() and free(). This was always an error; sp_sys_free() required a size, and all of our allocators implemented headers to track allocation sizes. The glue between them, the interface, simply omitted it.

In doing this, we were able to get rid of the headers that all the allocators had to implement. This drastically simplified the arena allocator, as well as fixed most of the longstanding issues that I have had with it. Likewise for the page allocator.

I also added some small things:

• test: Updated the tracking allocator used for leak testing to now verify what was actually allocated and find mismatches
• io: Added sp_io_dyn_mem_writer_take_str(), which reallocates the buffer so its length and capacity match, allowing it to be returned as a string which can be freed without the writer.
• io: sp_io_read_file() shrinks the buffer when a file reads short
• mem: Unrelated overflow checks in the arena
• mem: Arenas no longer crash on OOM
• test: Rewrote arena tests declaratively
• test: Added tests for the fixed allocator