NB: The 'z' size modifier is in C99.

Provide an alternate interface to the arena allocator to avoid wasting time on unnecessary calls to memset()

Rationale: "Basic allocation" refers to things outside of parsing proper,
mostly initialization. If such allocations fail, the system is globally
emory-starved from which it will likely not recover by returning failure.
In this case, terminating the process is in fact the most robust strategy as
it may mean the difference between a permanent hang and a temporary crash.

Cleaned up some memory leaks, got rid of gsequence, improved test macro to free memory that it allocated