This gives noticeable result for parsing simple input (modelled on 32-bit
unix port):

Before:
>>> micropython.mem_total()
3360
>>> micropython.mem_total()
4472

After:
>>> micropython.mem_total()
3072
>>> micropython.mem_total()
4052

However, effect on parsing large input is much less conclusive, e.g.:

Before:
>>> micropython.mem_total()
3376
>>> import pystone_lowmem
>>> micropython.mem_total()
33006

delta=29630

After:
>>> micropython.mem_total()
3091
>>> import pystone_lowmem
>>> micropython.mem_total()
32509

delta=29418