[Runtime] Enhancement: support pooled allocation of DemiBuffer

[kyleholohan]

Adds a few new classes to support pools of memory for DemiBuffer: MemoryPool which is a generic homogeneous pool of memory, and BufferPool which integrates MemoryPool and DemiBuffer. The latter is pretty light-weight: it's mainly to prevent exposing the details of MetaData to the user. Still needs some more testing, and a better DemiBuffer::clone implementation.