varray is allocator-able class now. This should allow us to handle externally-managed memory. Do we need StorageRef<> anymore?
Improved StorageRef: added size(), conversion to nonconst storage_type.
Added (modified) Toru’s stack allocator.
Fixed a few bugs remaining from the allocator work.
Misc changes to Tensor: added checks against TensorView-like hacks; improved move-type functionality.
Updated synopsis with more customization examples (e.g. swap storage backend, including allocators).
Minor cleanup.
Merge pull request #21 from evaleev/master

sync with evaleev/BTAS: varray is allocator-able class now.