feat(frontends/lean): provide position to parse_fn external function, add 'by' expression

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(kernel/abstract): propagate tags when abstracting

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(frontends/lean): propagate position information

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(kernel/converter): bug in is_def_eq

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
test(lean): add 'let' expression test

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
refactor(kernel): add mk_local function that has only two arguments

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(library/placeholder): allow types to be attached to placeholders

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(frontends/lean/builtin_exprs): add 'have' and 'show' expressions

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