Skip calling configure if there are no options

[eregon]

• Set all defaults in the constructor.
• Order the instance variables in the same order as in #configure.

Unfortunately because of JSON.dump_default_options this fast path won't apply for JSON.dump.
I wish those values (max_nesting: false, allow_nan: true) would be the default for all generate/dump-like methods and then they could be default in the State constructor.

[eregon]

Maybe we could store the default state for JSON.dump in a constant and then .dup it, to avoid calling configure which seems particularly slow/heavy.
Kind of similar to the current const_set :FAST_STATE_PROTOTYPE, create_fast_state.
However that wouldn't respect mutations (JSON.dump_default_options[...] = ...) or reassignments (JSON.dump_default_options = { ... }).
If we don't support direct mutation and freeze the Hash then we could still compare JSON.dump_default_options by identity and cache.

Though obviously any global state seems bad and maybe the best is to deprecate JSON.dump_default_options entirely.

[byroot]

Yeah, I toyed with this idea too last week, and JSON.dump_default_options is indeed the blocker.

I did however managed to avoid allocating the State object entirely in #662 so it's less of a problem now.