Rewrite let-args to object-centric model

[shirok]

As let-args becomes more featureful, the macrology gets pretty complicated. Specifically, plural option handling requires extra branch of code variation. I don't like it. We can rather delegate handling of variations to runtime <option-spec> arguments and keep macro simple.

The idea is to make <option-spec> hold the actual option argument value. As cmdline options are parsed, the values are accumulated in each instance. After parsing is done, let-args binds variables with the extracted values of each instance.

With this, the handling of plural options are done in ordinary runtime procedures.