Add ObjectMapper.createParser() and createGenerator() methods

[cowtowncoder]

Jackson 3.x will have direct JsonParser and JsonGenerator construction methods in ObjectMapper (and ObjectReader, ObjectWriter), as they are needed due to closer interaction between mapper and parser/generator.
But even in 2.x pattern of

JsonParser p = mapper.getFactory().createParser(....)

is quite common (ditto for generator). So it would probably make sense to add these methods both for convenience, and to offer upgrade path for code for eventual 2.x -> 3.0 migration, where direct access to JsonFactory, then calling on creation methods will not work as well (since it will not link read/write contexts properly).