Stream-to-stream synchronization at the buffer level

Highlights from README:

• Support for CUDA events, and Condition-based events for CPU backends.
• Overhaul of the backend interfaces, both user-facing but especially internal: full code sharing.
• Automatic stream-to-stream synchronization on a per-tensor-node basis.

Details from the changelog:

Added

• Interface files for Backends and Low_level.
• Fixed #245: tracking of used memory. But there's room for improvement.
• Stream-to-stream synchronization functionality, with lazy per-tensor-node synchronization.

Changed

• Migrated to cudajit 0.6.1.
• Verifying that code is linked with the right contexts, by tracking embedded_nodes with assignments.
• Renaming: (virtual) device -> stream, physical_device -> device.
• New files: split out backend_intf.ml, backend_impl.ml, schedulers.ml from backends.ml; moved Tnode.task to task.ml; renamed backend_utils.ml to c_syntax.ml.
• Removed half-static verification of merge buffer nodes inside device_to_device.
• Fixed #286: cross-stream-sharing incorporated into Tnode.memory_mode.
• Moved the multicore backend from a device = stream model to a single device model.
• Got rid of unsafe_cleanup.
• Rename subordinal to stream_id.
• Removed dependency on core, broke up dependency on ppx_jane.
• Huge refactoring of backend internal interfaces and API (not repeating same code).
• Built per-tensor-node stream-to-stream synchronization into copying functions.
• Re-introduced whole-device blocking synchronization, which now is just a slight optimization as it also cleans up event book-keeping.
• Simplifications: no more explicit compilation postponing; no more hard-coded pointers (all non-local arrays are passed by parameter).
• Fresh backends are now fresh modules to structurally prevent any potential cache leaking.

Fixed

• Validating merge nodes for the CUDA backend.
• Checking is_released on weak array retrieval.