Python: detect captured variables

[yoff]

Hacky PoC for captured parameters

• we added parameters to capturing function definitions.
  Capturing is detected via ScopeEntryDefinitions.
  These only exist for the inner-most nested function (see tests)
  We should make these exist at all levels to thread the values through.
  Right now, we cannot reach further nested functions (see m in tests).
• parameters currently need to be NameNodes,
  so we select the first mention inside the function body.
  That also only exists in the inner-most nested function
  (if that is the only one to mention hte captured variable)
  so this should be replaced with a synthetic node.
• we needed to extend ParameterNodeImpl
  and implement isParameterOf to make the data flow library connect them.
• we added arguments to calls to capturing functions
  (referencing the call graph)
• These should be current value of the variable at the call site
  This is currently approximated by the latest use before the call site
  which is further approximated by any variable of the same name in the same basic block as the call
  The right thing to do is to make the call a useand let SSA handle it.
• every captured variable has been given position -3
  this should be based on the variable name instead.
  That will be easy once we rebase on the new call graph PR,
  as it has genralised positions.