optimize sparql.Bindings

[FlorianLudwig]

This improves performance 13% on the rdflib-benchmark with the default (memory) store and a store size of 4k.

Moving straight to collections.ChainMap was considered but

1. Bindings behaves differently when one key is in multiple dicts, as they are iterated over multiple times and also counted on __len__ multiple times
2. The API is a bit different, increasing the changes size

I do think that the differences for __len__ and __iter__ are unintentional / bugs. If so, I would rework this MR to fix those.

[coveralls]

Coverage decreased (-0.05%) to 75.434% when pulling 5574acc on FlorianLudwig:sparql-speedup into dde9db8 on RDFLib:master.

[coveralls]

Coverage remained the same at 75.477% when pulling 07bc739 on FlorianLudwig:sparql-speedup into 6c294aa on RDFLib:master.

[nicholascar]

Test passing and this has been here for a while so I'll merge