Simplify expressions

[alukach]

When combining Expr structs, it is easy to produce expressions with redundant or superfluous logic. We should be able to use Expr.reduce to boil down an expression to its minimal form.

e.g.

cql2.Expr('"properties.private" = false') + cql2.Expr('"properties.private" = false') == cql2.Expr('"properties.private" = false')

cql2.Expr("true") + cql2.Expr('"properties.private" = false') == cql2.Expr('"properties.private" = false')