m_realloc: Account only allocation size difference in total_bytes_allocated.
Add new alloc metric: current_bytes_allocated.

Unlike total_bytes_allocated, this tracks m_free()'s too.
Add new alloc metric: peak_bytes_allocated.

This is just max value of current_bytes_allocated seen.
Make it possible to turn off collecting memory stats (MICROPY_MEM_STATS).
Rename default config file to mpconfig.h, and port's to mpconfigport.h.

mpconfig.h will automatically pull mpconfigport.h.
Merge pull request #41 from pfalcon/more-mem-stats

Collect more memory statistics