Change entries_tree() to take an offset

[philipc]

Also adds an entries_from() method that takes an offset, since we probably don't want to change the entries() API at this stage. I'm open to better names.

[coveralls]

Coverage increased (+0.01%) to 93.846% when pulling def5340 on philipc:entries-tree-offset into 53c8f11 on gimli-rs:master.

[fitzgen]

entries_from sounds pretty good. Could also do entries_at or entries_{at,from}_offset, perhaps?

[coveralls]

Coverage increased (+0.02%) to 93.854% when pulling 9058a66 on philipc:entries-tree-offset into 53c8f11 on gimli-rs:master.

[philipc]

Thanks, changed it to entries_at_offset.