libmem: implement policy-agnostic memory allocation/accounting.

[klihub]

This patch series

• implements a policy-agnostic library for memory allocation/accounting
• replaces existing memory accounting in the topology-aware policy with libmem
• plugs in libmem-based memory accounting and allocation to the balloons policy

For a more detailed description of the main ideas, core concepts, and some high level implementation details, see the included package documentation.

TODO items:

• [x] get #329 merged then rebase
• [x] add missing ensureNormalMemory() during initial zone selection
• [x] allow for both strict and preferred memory type selection
• [x] add an interface for overriding builtin zone expansion and overflow resolution
• [x] make sure memory type preference is consistently either a preference or a requirement
• [x] add a description of how the allocator works/describe the basic idea
• [x] comment/document zone expansion, zone overflow resolution
• [x] clean up debug-logging to be actually helpful

A few remaining questions related to this initial implementation:

• Should we allow more granular prioritization than the current QoS class/Preserving/Reservation mapped to Inertia ?
• Are the builtin defaults good enough ?