Optimize memory usage

[eigmax]

StarkContext takes huge memory space since we define the elements are from FieldExtension, but this is not a must.

• Using base field as more as we can, take ctx.cm1_n for instance, cm1_n can use the Goldilocks element. Hence, we need figure out the proper element type for the members in StarkContext.
• Drop const and cm PolsArray after using
• Use mmap, instead of vec![] to allocate memory. Use memmap2, and setup MmapOptions::map_anon() to write arbitrary size data.