Summary:
OCaml 4.12 added
```
val __FUNCTION__ : string
__FUNCTION__ returns the name of the current function or method,
including any enclosing modules or classes.
```
This diff simplifies ppx_trace using `__FUNCTION__` to obtain the name
of the function containing each call to a `Trace` function. Before
this diff this is done by maintaining a stack of function names
obtained by parsing value binding patterns during preprocessing. This
technique is not entirely robust and does not deal with some cases (if
calls to `Trace` functions appear in some places, preprocessing fails
with an exception).
Reviewed By: jvillard
Differential Revision: D27396915
fbshipit-source-id: da7aa2945
Josh Berdine
6ad7bbe7f1