Add transformer primitives as methods on TransformerContext

[maffoo]

This is a proposal to make transformer_primitives functions available as methods on TransformerContext. Then they can be invoked without passing extra parameters which are already stored on the context, such as the deep flag and tags_to_ignore. I think this would be simplified somewhat if we changed transformer primitives to take TransformerContext instead of Optional[TransformerContext], with a default value specified as appropriate, but we can't change that due to backwards compatibility so there are a bunch of places where we would have to do things like context = context or transformers.TransformerContext(...) to instantiate a default context if needed. Still I think this simplifies existing transformers quite a bit and could prevent bugs in the future from forgetting to pass args from the context manually. I also added a function TransformerContext.replace that allows creating a new context with some parameters changed, to allow overriding some parameters before calling a primitive. I've updated a few existing callsites to use the new style, but for now just want to put this up to get feedback before changing more.