Update in toString

[Romain Manni-Bucau <rm...@gmail.com>]

Hi guys,

I updated toString of jsonobject/jsonarray to be based on jsongenerator to
avoid surprises and ensure we reuse all the goodness we can put in the
dedicated API (using StringWriter).

It has one big impact: BufferProvider of char[] must be almost everywhere
and our jsonb impl is no more portable (regarding jsonp). We can duplicate
this API with a nice default when running johnzon-jsonp under the hood but
wonder if we still care about being portable or if it really never happens
and we can drop that feature.

wdyt?

Romain Manni-Bucau
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