Implement WeaklyDivisibleSemiring for BooleanWeight

[bluebear94]

According to the definition in the docs for WeaklyDivisibleSemiring, BooleanWeight (S = bool, + = |, * = &, 0 = false, 1 = true) should be weakly divisible: there are three combinations (x, y) such that x + y != 0:

• x = true, y = true: in this case, we can set z = true so that true = (true | true) & true.
• x = true, y = false: we can set z = true so that true = (true | false) & true.
• x = false, y = true: we can set z = false so that false = (false | true) & false.

In fact, BooleanWeight is divisible because the inverse of true is true.

This PR adds an implementation of WeaklyDivisibleSemiring for BooleanWeight.