Use OptionInfo type for option registration args/kwargs.

[benjyw]

The options registration system has for a long time passed around
the naked args and kwargs used at registration time.

However the more recent implementation of class field-based
declarative options introduced an OptionsInfo dataclass to
encapsulate that data.

This PR spreads the use of OptionsInfo into the options
registration code. Advantages include:

• More succinct code.
• Proper type annotations.
• No confusion about when to use * or **.

Note that this change renames OptionsInfo to OptionInfo,
since it represents the registration of a single option.
It also renames its fields to args and kwargs, since that
is the usage in most of the related code. Also, the name flag_options
was doubly confusing: A) A flag is just one aspect of an option, and
B) "options" is overloaded. This refers the knobs you can set when
registering an option, so using the word "options" for it is brain-hurting.
I considered "knobs", but figured "kwargs" was just as good, and already
in use.

[benjyw]

Review note: The diffs are pretty trivial, even though over 100 lines are touched.

[huonw]

Makes sense, looks like a good improvement.

[benjyw]

It should come as no surprise that this was also good prep work for my continued chipping away at the OptionsBootstrapper... :)