safe, ref-counted support for dynamic memory allocation

See this thread:

http://codespeak.net/pipermail/cython-dev/2008-November/003090.html

This depends on ticket http://trac.cython.org/ticket/152 as the Memory object should subtype `PyVarObject` to be efficient. An initial implementation could do without, though, and simply do two allocations.

Migrated from http://trac.cython.org/ticket/153

Start by reading the linked Cython-dev thread and ticket 152, then compare their requirements with the migrated ticket. The issue does not name implementation files or tests; completion would require a decided design for safe, reference-counted dynamic allocation and its relationship to PyVarObject, including whether the two-allocation approach is acceptable.

Written by the indexing model from the issue text.