docs(core.utils): publish ProgramCacheResource and persistent backends

Add the new cache ABC, the two concrete backends, and the key helper to
the cuda.core.utils API reference.

Part of issue #178.

Author: cpcloud

cuda_core/docs/source/api.rst

@@ -271,7 +271,18 @@ Utility functions
    :toctree: generated/
 
    args_viewable_as_strided_memory
+   make_program_cache_key
 
    :template: autosummary/cyclass.rst
 
    StridedMemoryView
+
+Program caches
+--------------
+
+.. autosummary::
+   :toctree: generated/
+
+   ProgramCacheResource
+   SQLiteProgramCache
+   FileStreamProgramCache