refactor(kernel/expr): for_each and find functional objects

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
refactor(kernel/expr): for_each_fn, replace_fn, and find_fn without templates

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
refactor(kernel/free_vars): cleanup free_vars procedures

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