termType values case confusion

[timbl]

The original termType values like 'symbol' 'bnode' and 'collection" and 'formula' were lower case.
The common RDF api work produces Inital Cap names like Named Node. This inconsistency led to least least one bug and is confusing to the developer.

Change all to inital caps -- this involves changing all 'collection' to 'formula'
This is a large change.

[elf-pavlik]

capturing from gitter chat https://gitter.im/rdfjs/Representation-Task-Force?at=58c20004e961e53c7fc4743e

It looks like Collection and Formula do NOT extend Term so IMO they should NOT use .termType. It also seams that on low level collection gets represented with Quads #169 and Formula pretty much represents a Graph. Possibly more related to https://github.com/rdfjs/representation-task-force/issues?q=label%3A%22HighLevel+API%22