This allows the wrapper functions in the main open3 to be defined
while using our ProcessBuilder logic for the internal popen
implementation.

Note this adds logic to reject redirects from a numeric fd to a
live IO object (or not a String or to_path object) since we cannot
support direct IO redirects with ProcesBuilder.

This patch allows tests to complete with the ProcessBuilder impl.
Only three tests fail:

* test_numeric_file_descriptor2 and test_numeric_file_descriptor2
  fail due to redirecting streams to a pipe IO.
* test_pid fails expecting a real PID which we cannot provide via
  ProcessBuilder.

https://github.com/ruby/open3/commit/73f986c233