add Value.considered_bool() function

add Value.considered_bool() function

testing len(Value) == 1 is one of the rare places that makes an assumption
between Type 1 (Ast) low-level nmigen language constructs and
between Type 2 (dsl.Module) high-level nmigen language constructs.

a "Value.considered_bool()" function de-couples that assumption and allows
for a proper separation between Type 1 and Type 2 nmigen language
constructs, providing a clean design where Liskov Substitution Principle
may successfully be applied at the Type I (ast) level