Improve validator performance - early returns

[dholroyd]

In translating Haskall pseudo code from An algorithm for RELAX NG validation into Rust, some spots where Haskall lazy evaluation lead to efficiencies are turned into eager evaluation in the Rust - we evaluate schema expressions we don't need to.

For example we don't need to calculate the derivative of p2 if we already know that p1's derivative became NotAllowed here:

relaxng-rust/relaxng-validator/src/lib.rs

Lines 1016 to 1020 in 90b55c4

	let d1 = Self::att_deriv(memo, p1, schema, att);
	let s1 = schema.group(d1, p2);
	let d2 = Self::att_deriv(memo, p2, schema, att);
	let s2 = schema.group(p1, d2);
	schema.choice(s1, s2)