Symbolic{Bool} and simplification

[shashi]

Allow symbolic conditions in rules:

@rule abs(~x) => ~x >= 0 ? ~x : -~x
# In a Bool ruleset:
@rule ~x >= 0 => isnonnegative(~x, @CTX) # isnonnegative returns Union{Bool, Nothing}

This should be lowered such that if the truth value of ~x >= 0 cannot be ascertained, then it would leave the input expression unchanged.

We can potentially allow expressions with if conditions. Maybe just by adding an ifelse method.