fix(library/hop_match): typo

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(library/simplifier): move to locally nameless approach in the simplifier. Contextual simplification may add rewriting rules with free variables, and it is a mess to manage them when using de Bruijn indices

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
test(tests/lean): new simplifier test

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>