Vector-space arithmetic requirements.

Implements vector space arithmetic operators (uppercase letters are for vectors, lowercase letters are for scalars):

• Vector-additive: V += U, W = V + U, V += U, W = V - U;
• Scalar-additive: V += a, V = U + a, V = a + U, V -= a, V = U + a, V = a - U;
• Scalar-multiplicative: V *= a, V = U * a, V = a * U, V /= a, V = U / a;
• Incrementation if enabled (for integral value types by default): V++, ++V, V–, –V.