topdown/parse_units: Use big.Rat for units parsing. (#4857)

Previously, we used big.Float for units parsing, but this resulted in
occasional rounding issues, due to values like 0.001 not being perfectly
representable in floating-point format.

This issue was fixed by switching units parsing to use big.Rat, since
Rationals can generally handle such quantities with perfect precision.

Fixes #4856.

Signed-off-by: Philip Conrad <philipaconrad@gmail.com>