Add a medium length string mode

[ngoldbaum]

This adds a "medium string" mode where the sizes of arena allocations are stored using a char instead of a size_t, saving 8 bytes per string for strings with lengths between 16 and 255 bytes. I modified the tests so they use longer strings more often which exercises all the different memory allocation modes we're now supporting.

I also dropped the attempt to store the size_t sizes in aligned locations because it adds a bunch of bookkeeping headaches. If it turns out the memcpy to unaligned addresses is slow we can come back to this.

[ngoldbaum]

I'm glad I have ASAN running in the tests here, it noticed a serious flaw in the first version of this.