Don't search in path if filename contains a '/', even if it is an "implicit" filename such as "foo/bar".

Use the system-provided execvpe() if possible.  Otherwise, use
a serious reimplementation written in OCaml and patterned after
the Glibc execvpe() implementation.

Added a test in tests/lib-unix/unix-execvpe.

Don't test Unix.execvpe if we are using the system-provided implementation.
The execvpe() functions provided by Win32 and Cygwin aren't quite to
our specs.  At any rate, the test is there to find bugs in our
implementation of execvpe(), not in others's.

travis: test 'make clean'

Should the "clean: defaultclean" default rule be part of
  testsuite/makefiles/Makefile.common
?

(no change entry needed)

* Fix duplicates in Cmmgen when handling switches with no default and not all cases

* Improve handling of incomplete Lambda switches in Flambda

* Add test (for reference) and changes

* Fix nitpick

* Cleanup: split the switch stores to reflect usage

* Improve compilation of incomplete switches with flambda

* Split switch stores into context-aware and -unaware versions

* Credit reviewers

* Go back to a single Switch.t_store type

(cherry picked from commit fd7f18f8d35aedc932c174967366837f37f3100d)

afl-fuzz instrumentation fix for classes

PR#7363: documentation, start heading levels at 1