Fixing several issues with malformed triggers

[marcoeilers]

Fixing viperproject/carbon#489 by:

• marking equality expressions as not allowed in triggers
• fixing the check for which quantified variables are contained in an expression for both trigger inference and trigger consistency checks, which was previously incorrect in the presence of let-expressions. In particular, let x == (y) in e was considered to contain variable y even if e does not contain any reference to x (or y itself).