do not subclass list

Not subclassing list should allow us to bypass some of the GC machinery associated with them.  In the end, the benchmarks are roughly the same.  To improve this, implement something along the lines of the FREELIST cache in listobject.c.  The idea is to keep a stash of lists/hashseqs around to avoid repeated memory (de)allocations.

The main challenge would be in determining how many lists/hsqs of how many sizes to keep around for a given cache size.  Since the benchmarks involve repeated calls of the same argument length, they may not show any speedup.