Use an enum instead of a boolean to indicate the closed-ness of a fields constraint in function parameters

[popematt]

#218 added a boolean paramter to indicate whether fields are closed or open. In the interest of readability, we should change this to an enum before releasing v1.0.0 of ion-schema-rust.

Note that this is specifically calling out function parameters/arguments. It's fine to have a boolean as the return type of is_closed() because the function name describes the return value. However, something like fields(my_fields_map, true) has nothing to indicate the meaning of the boolean, whereas fields(my_fields_map, FieldContent::Closed) is very clear.

See e.g. https://blakesmith.me/2019/05/07/rust-patterns-enums-instead-of-booleans.html