Inconsistent mutable reference tracking in vector module.

[metasim]

The following methods mutate the underlying C types but do not require a mutable reference in Rust.

• LayerAccess::create_defn_fields
• LayerAccess::set_feature
• FieldDefn::set_width
• FieldDefn::set_precision
• Feature::set_field_string
• Feature::set_field_string_list
• Feature::set_field_double
• Feature::set_field_double_list
• Feature::set_field_integer
• Feature::set_field_integer64
• Feature::set_field_integer64_list
• Feature::set_field_datetime
• Feature::set_field

Also:

• FieldDefn::add_to_layer mutates the layer parameter
• Feature::create mutates lyr parameter

This means the following code compiles, but shouldn't under Rust ownership rules:

#[test]
fn multiple_mutable_references() {
    let f = FieldDefn::new("foo", OGRFieldType::OFTInteger).unwrap();

    let f1 = &f;
    let f2 = &f;

    f1.set_width(10);
    f2.set_width(20);
}