Implement the ItemsView protocol for component instances. Closes #237.

[AdamHillier]

This implements most of ItemsView for components, which means that components can be iterated through, yielding pairs of (field_name, field_value). Importantly, this view is a flattened representation including the fields of all sub-components.

As a result, dict(component_instance) is a meaningful construction. To illustrate, here is an example of dict(component_instance) for a component with a nested sub-component:

{
    "a": 10, # Normal field
    "b": (5, -42.3), # Normal field
    "child": "namespace.filename.Child", # Component field - the qualified name of the class that's being used
    "child.b": (-1, 0.0), # Normal field (of sub-component)
}

In particular, this dict is the precise, minimal configuration needed to completely re-create the component, e.g. by doing:

config_dict = dict(old_instance)

new_instance = ComponentClass()
configure(new_instance, config_dict)

The above will work even if default field values have changed since the old instance was created.

[AdamHillier]

Technically this is a breaking change, but I strongly doubt that anybody was using component classes with custom __len__, __contains__, or __iter__ methods.

I'm happy to make a new minor version release but I think we can be slightly flexible about semver here and not worry about a new major version.

[AdamHillier]

This was a bug, now fixed.

[lgeiger]

Awesome! This is great 🚀