templater: promote tomap() to an interface type I originally considered merging tomap() with itermaps()/getmember(), but decided to not. We might want to add support for chained map operations (e.g. {foo % func() % ...}), where func() will return a mappable object, and 'foo % func()' will be a mappedgenerator of mappable objects.

templater: rename mappable to hybriditem as it is the primary use case This frees up the name 'mappable' for new interface type.