Code coverage updates (#50)

* code coverage updates

- python support
- refactored source

* remove code_coverage::operator+ and operator+=

* impl/coverage.hpp

source/python/omnitrace/__init__.py

@@ -35,6 +35,7 @@ This submodule imports the timemory Python function profiler
 """
 
 try:
+    from .libpyomnitrace import coverage
     from .profiler import Profiler, FakeProfiler
     from .libpyomnitrace.profiler import (
         profiler_function,
@@ -65,6 +66,7 @@ try:
         "config",
         "profile",
         "noprofile",
+        "coverage",
     ]
 
     import atexit