More typesafe >, <, = comparisons

[sviezypan]

The bug was that compiler did not complain when user compared 2 values of different types:

select(customerId, fName, lName, verified, dob).from(customers).where(fName === true)
//or
select(customerId, fName, lName, verified, dob).from(customers).where(verified === "hello")

The problem was that in Expr where === is defined

    def ===[F2, A1 <: A, B2 >: B](that: Expr[F2, A1, B2]): Expr[F :||: F2, A1, Boolean] =
      Expr.Relational(self, that, RelationalOp.Equals)

When B =:= Boolean and B2 =:= String, Scala happily infers B2 to be Any, so that would compile.

This PR introduces ComparableTypes trait which would prevent that behaviour.
We still allow to compare different numeric or date types like 3 < 5.5 etc.

[jczuchnowski]

Suggested change

	"As those are unrelated types, this query would blow up at database level."
	"As those are unrelated types, this query would fail at database level."