Add support for nested py_capture_output() calls.

[t-kalinowski]

This adds support for nested py_capture_output() calls. I.e.,

py_capture_output({
  py_capture_output({
    py_run_string("print('hello')")
  })
})

See the included tests for a fuller example.

The issue with the previous implementation was discovered while investigating a segfault in the Keras testsuite, which would in some situations setup multiple py_capture_output() contexts. When that happened, the reference to the original sys.stdout would be lost permanently, resulting in no further output, and a cryptic R error message each time flush_std_buffers() was invoked from C++ functions or from the R top-level task callback.