I made some more updates to keep up to date with the spec. I'll wrap up this PR next week and possibly just postpone the implementation of iterate to another PR.

Closes #1288, closes #1302, closes #1303

@jpivarski this PR is ready for review now. Here are some highlights/comments:

• I fixed the __len__ vs num_entries issue from RNtuple __len__ should return the number of keys, and have a num_entries property for the number of entries #1220.
• I implemented a simple iterate method. It supports step sizes in terms of memory, but the current estimation method might not be optimal.
• I added an RNTupleField class, so that now you can do things like obj["field"].array() and obj["field"]["subfield"].array(), but I haven't implemented path support yet for things like obj["field/subfield"].array().
• I updated some things to match the spec, including fixes to RNTuple fields have unexpected 4 bytes when recent master ROOT version is used #1288 RNTuple has added a new fMaxKeySize in Anchor #1302 RNTuple has removed meta data from Footer #1303.
• I made some other minor improvements like adding a __repr__ method for better printouts.