Determine truth of reduction expressions and defer Context mutation

[Charles-Johnson]

New features:

// Determine the truth of a reduction
assert_eq!(context.execute("a (-> d)"), "true");
assert_eq!(context.execute("d (-> a)"), "false");

// A concept never reduces to itself
assert_eq!(context.execute("a (-> a)"), "false");

// Cannot reduce a reduction expression between unrelated concepts
assert_eq!(context.execute("d (-> f)"), "d (-> f)");
// Can ask whether a reduction is true or false
assert_eq!(context.execute("(a (-> d)) (-> true)"), "true");
assert_eq!(context.execute("(d (-> a)) (-> false)"), "true");

Internal changes:

• replaced try! with ?
• for most Context methods, replaced &self with &self, &[Self::Delta] and &mut self with &self, &mut self Vec<Self::Delta> to defer mutation of selfuntil execute method
• refactor with more standard library features (methods for Option, Result, Vector, Iterator)