[Cherry-picked e896db2]

[Cherry-picked da42229]

[Cherry-picked 0b4e677]

[Cherry-picked b34df4d]

[Cherry-picked 2405109]

TypeComparer.explaining is like TypeComparer.explained, but instead of
just returning the trace, returns the result, still allowing the trace
to be accessed via .lastTrace, as exemplified by implementing
TypeComparer.explained in terms of TypeComparer.explaining.

Also add, but leave commented out the call of, a trace.dumpStack, which
is like Thread.dumpStack(), but outputing to System.out, like all our
tracing does - so the two don't interact when unbuffering onto the
terminal.  Also, we can do customisations like filtering out stack
elements, limiting the stack.

[Cherry-picked 5aaea2f][modified]

But keep the extraction of the instDecision logic, and keep the tests
cases I used in studying this change.

Also simplify and update InstantiateModel.  Martin had tweaked my i14218
fix to make it more conservative: in the (NN, UU) case (i.e. no inferred
bounds, only a UU upper bound declared) for covariant type parameters,
revert back to minimising to Nothing rather than maximising to the
declared bound.

[Cherry-picked 43fc10c][modified]