topdown: misc changes to units.parse_bytes (#3672)

* topdown: port units.parse_bytes() tests to external yaml tests
* topdown/units.parse_bytes: add test for lowercase/quotes behaviour
* topdown/units.parse_bytes: misc improvements
* using int64, and the big.Int, big.Float types consistently
* replaced use of deprecated registeration function

Signed-off-by: Stephan Renatus <stephan.renatus@gmail.com>