Implement knob for tweaking handle output buffer
Implement knob for tweaking handle output buffer

- Rename[^1] "buffer" open arg to "out-buffer", as we'll be adding
    another knob for input buffer. The "buffer" will remain under
    deprecation warning for 3 releases (it was available for
    just 1 release)

[1] https://irclog.perlgeek.de/perl6-dev/2017-10-15#i_15307167

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