Enhance Rel to make rewrites easier

[jacques-n]

Let's add the following functions to plan.Rel along with impls in the associated impls.

// Copy creates a copy of this relation with new inputs
Copy(inputs ...Rel) (Rel, error)

// GetInputs returns a list of zero or more inputs for this relation
GetInputs() []Rel

// CopyWithExpressionRewrite rewrites all expression trees in this Rel. Returns nil 
// if no changes were made, otherwise a newly created rel that includes the given expressions
CopyWithExpressionRewrite(inputs ...Rel, func(expr.Expression) (expr.Expression, error)) (Rel, error)

[jacques-n]

Resolved by #29