dynamic type function variants for Variants and Values

[arcnmx]

Addresses the main part of #507 by providing explicit dynamic type variants of:

• Variant::array_from_iter -> Variant::array_from_iter_with_type
  (a mouthful but consistent with other existing functions like from_data_with_type_trusted)
• Variant::from_maybe becomes Variant::from_none and Variant::from_some
  • and adds an accessor: Variant::as_maybe
    (panicking, should there be a try_as_maybe? should it return an Option<Option<Variant>> to be more in line with Variant::as_variant?)
• Variant::is -> Variant::is_type
  (consider is_of_type instead?)
• Value::is -> Value::is_type (ditto)
• Value::transform -> Value::transform_with_type
  (consider transform_into/transform_to_type instead?)

[sdroege]

CC @cgwalters @lucab in case you have opinions

[lucab]

Generally speaking, these new methods looks useful to have 👍.

[lucab]

I may have misunderstood the logic, but I'm not exactly sure that this matches what the docstring says.
Let's maybe cover this method with a test to avoid any doubts?

[arcnmx]

A maybe variant is a container with either 1 or 0 values, thus roughly the logic for this function is if self.n_children() == 0 { None } else { Some(self.child_value(0)) } - Variant::try_child_value already exists as a rough equivalent to rust's slice.get(0) for this purpose, so it didn't seem worth duplicating the logic here.
However, I really should just use ffi::g_variant_get_maybe instead, since it's clearer - it implements the exact same logic regardless.

Good point on adding a test for this method though, will do!