Reworked memory allocation and compiler warnings

[stawiski]

My work as per our discussion under #56

[mortie]

I wonder, maybe a simple way to drop in a custom malloc and free, in addition to handling malloc returning NULL, would be better? You could then make an allocator which just gives memory from a pre-allocated buffer and returns NULL when out of space in that buffer. It would certainly keep tinyexpr much simpler.