feat: add parameters

[jpivarski]

No description provided.

[jpivarski]

Although parameters in Awkward Array (Python) are a mapping from arbitrary names to arbitrary JSON-serializable data,

• they're usually empty
• when not empty, it's usually just __array__, which can take a short list of possible values: "string", "bytestring", "char", "byte", "categorical", "sorted_map"
• when it's not __array__, it's either __list__, __record__, or __doc__, which can only take string values
• or it's something else; a general case (extremely rare).

So our struct Parameters will handle the common cases efficiently, and the uncommon cases... maybe an ImmutableDict{Any}.