feat!(core): Change Signal to be a sumtype

We want to be able to reason about if a signal is empty, constant, or sampled
at compile time without using any trait objects. Moreover, the core Argus
library shouldn't care about how it deals with interfacing with other languages
like Python. Thus, we remove the need for having an `AnySignal` type and what
not.

argus-core/src/prelude.rs

@@ -1,3 +1,3 @@
 pub use crate::expr::{BoolExpr, Expr, ExprBuilder, ExprRef, NumExpr};
-pub use crate::signals::{AnySignal, ConstantSignal, Signal};
+pub use crate::signals::Signal;
 pub use crate::{ArgusError, ArgusResult};