micropython-specific call to pre-allocate object to a given size #917

Here's the idea I had for a long time, remember at right time to record it: to fight memory fragmentation, etc. it would be nice to have call like:

obj = micropython.alloc(type, 10)

which would create a container object which could hold up to 10 items without reallocations. Obvious candidates for type are list and dict, and support for more types can be added as makes sense.

Note that we still can stay compatible with other Pythons by providing shim:

def alloc(type, sz):
    return type()