Break OvertypedType -> BlockVariable -> OverloadedType reference cycle

[jrmaddison]

Use a weakref for OverloadedType.block_variable, accessed indirectly via a property. The result persists for as long as it is used elsewhere (e.g. on the tape, referenced by a Control, etc).

Example use:

ot.create_block_variable()
object_0._block_variable = ot.block_variable
object_1._block_variable = ot.block_variable

First line creates and then immediately destroys a BlockVariable. Second line creates another BlockVariable, holding a reference to it, so that the third line references the same BlockVariable. Net effect: attach a new BlockVariable to ot, hold two references on object_0 and object_1, hold a weak reference on ot.

Could be optimized by adding an OverloadedType.clear_block_variable method to replace the first line of the above example

def clear_block_variable(self):
    self._block_variable = lambda: None

which avoids the creation and immediate destruction of a BlockVariable.