This commit removes fields from the responsibilities of `FlowVar.qll`.
The treatment of fields in that file was slow and imprecise.
It then adds another copy of the shared global data flow library, used
only to find local field flow, and it exposes that local field flow
through `localFlow` and `localFlowStep`.
This has a performance cost. It adds two cached stages to any query that
uses `localFlow`: the stage from `DataFlowImplCommon`, which is shared
with all queries that use global data flow, and a new stage just for
`localFlowStep`.
