Support multiplication of units

[thegedge]

We previously supported multiplication between Measurables, but specifically the inner product:

(Measured::Length(2, :cm) * Measured::Length(3, :cm)).to_s
> 6 cm

This is convenient, but not (mathematically) correct. When multiplying two measured values we should combine units from the same system, raising the exponent, and then multiplying/dividing the other units:

Measured::Length(2, :cm) * Measured::Length(3, :cm)
> 6 cm^2

Measured::Length(2, :m) * Measured::Length(0.002, :km) / Measured::Time(0.1, :seconds)
> 40 m/s

This change should not come at a performance penalty for any current use cases:

• unit + unit
• unit.scale(scalar)
• unit.convert_to(:simple_unit)
• etc