Make JsonGenerator.writeXxx() methods chainable

[cowtowncoder]

Currently Jackson 2.x has void as return type for write methods, like:

public abstract void writeStartArray() throws IOException;

which prevents chaining of write calls. It would be nice to instead have these return JsonGenerator, but change of return type is bytecode-/binary-incompatible change and as such absolutely no-go for 2.x.
But for 3.x this would be doable. It

One possible open question here is that format-specific subtypes' overriding of write methods; should they change return type to proper "this" type? I don't want to make JsonGenerator (... misnomer at this point, but it is what it is) type-parameterized.
I guess this is not a problem for most usage but should probably decide on recommended approach.

[anbusampath]

which prevents chaining of write calls.

Chainable calls are nice features.