Add doc.retokenize() context manager

[honnibal]

This patch takes a step towards #1487 by introducing the
doc.retokenize() context manager, to handle merging spans, and soon
splitting tokens.

The idea is to do merging and splitting like this:

with doc.retokenize() as retokenizer:
    for start, end, label in matches:
        retokenizer.merge(doc[start : end], attrs={'ent_type': label})

The retokenizer accumulates the merge requests, and applies them
together at the end of the block. This will allow retokenization to be
more efficient, and much less error prone.

A retokenizer.split() method will then be added, to handle splitting a
single token into multiple tokens. These methods take Span and Token
objects; if the user wants to go directly from offsets, they can append
to the .merges and .splits lists on the retokenizer.

The doc.merge() method's behaviour remains unchanged, so this patch
should be 100% backwards incompatible (modulo bugs). Internally,
doc.merge() fixes up the arguments (to handle the various deprecated styles),
opens the retokenizer, and makes the single merge.

We can later start making deprecation warnings on direct calls to doc.merge(),
to migrate people to use of the retokenize context manager.

Checklist

• I have submitted the spaCy Contributor Agreement.
• I ran the tests, and all new and existing tests passed.
• My changes don't require a change to the documentation, or if they do, I've added all required information.
  (For now it's backward compatible, so no docs changes required yet --- when we deprecate, we need to update the docs)