Add an option to pattern match using structural equality instead of definitional equality.

[abdoo8080]

Add a way to opt into pure structural Expr equality for ~q-style pattern matching instead of definitional equality. Currently ~q silently unfolds definitions and performs expensive reductions when matching, which in my benchmarks is over 100× slower than a straightforward AST comparison. Because I only need exact structural matches in my use case, it would be helpful to have either a new macro (for example ~q!) or a configuration flag that skips definitional checks and only compares expressions directly. This change would let users opt in to a much faster matching mode whenever unfolding isn't required.