topdown: Add units.parse builtin (#4676)

This function works on all base decimal and binary SI units of the set:

    m, K/Ki, M/Mi, G/Gi, T/Ti, P/Pi, and E/Ei

Note: Unlike `units.parse_bytes`, this function is case sensitive.

Fixes open-policy-agent#1802.

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