The goal is to provide greater control of input, output and error
streams of the interpreter. It is now possible to specify those
as options when creating a new interpreter. The provided values
are propagated to relevant stdlib symbols (i.e fmt.Print, etc).
Care is taken to not update the global variables os.Stdout, os.Stdin
and os.Stderr, as to not interfere with the host process.
The REPL function is now simplified. The deprecated version is removed.
The tests are updated to take advantage of the simplified access
to the interpreter output and errors.
Fixes#752.
