Implemented per-sandbox context-sensitivity.

Dataflow facts in SOAAP are Value* -> int maps whereby the latter
represents the former's taint state. The dataflow algorithm initialises
the taint for annotated variables and then iteratively propagates this
along use-def chains performing joins until a fixed point is reached.

This has the limitation that the taint state computed for each Value
will represent the combined state for all Sandboxes, leading to
imprecision for Value*'s used by multiple Sandboxes (e.g. global vars,
variables in a method executed by multiple sandboxes, ...).

This commit adds context-sensitivity whereby we now keep dataflow
state separate for each Sandbox. This leads to better precision and
more accurately models run-time behaviour.