Auto merge of #240 - Noughmad:length_mul_scalar, r=nical
Auto merge of #240 - Noughmad:length_mul_scalar, r=nical

Scalar multiplication and division for Length

Implement `Mul<T>`, `Div<T>`, `MulAssign<T>` and `DivAssign<T>` for `Length<T, U>`
which multiply or divide the value but preserve the unit.

Tests for all four new implementations are included.

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servo/rust-geom