Add --statements flag to juvix dev latex export

[janmasrovira]

This pr adds the --statements flag to juvix dev latex export. It can be used like this:

juvix dev latex export --statements "List; elem; find" List.juvix

All statements are selectable by their 'label'. The label of a statement is defined as:

1. The function/type/operator/alias/axiom/local_module/fixity/iterator name being defined.
2. The module name being imported.

Limitations:

1. Only top statements are allowed. I.e. statements inside local modules cannot be selected.
2. Open statements are not selectable. Giving --statements "My.Module" will refer to the import statement import My.Module.
3. Single constructors are not selectable. Only the whole type definition.
4. No comments will be printed.