Support Symbols for Hash keys

[mperham]

json/add/core does not support Symbols for Hash keys today. This is a major issue for downstream systems like Sidekiq which rely on JSON for serialization because we can't transparently serialize the full range of method arguments like keywords.

I'd like to propose a way to support Symbol'd keys by "smuggling" the type in the String itself:

{
  :mike => 123
}

{
  "_s:mike": 123
}

This would allow Sidekiq users to use keyword arguments in their jobs without any issues. What do you think?

See also sidekiq/sidekiq#6209

[mperham]

And I would be perfectly ok with this being opt-in: JSON.generate(..., type_smuggling: true)