Math with SI & imperial mix don't convert in some cases

[Vindaar]

Reported by @arkanoid87 on matrix/discord:

import unchained

let x = 5.m
let y = 3.ft⁻¹

echo x * y
# results in
# 15 Meter•Foot⁻¹

As both are length units, ft should be auto converted to SI.

[Vindaar]

Fixed via #30.