Put common probe and session context state definitions in their own files.

Put common probe and session context state definitions in their own files.

translate.cxx contained code to produce the common probe and session
context state as C code snippets. Large parts were not dependent on the
session state at all, so they really were just static blobs of C code
wrapped in c++ io stream operators. These have been put in their own
C runtime header files (common_session_state.h, common_probe_context.h
and runtime_defines.h) to make it easier to edit and document.