Improve pprinting of stateful examples

[tybug]

singleton_printers does unconditional id-based replacement, which is not always correct (see tests). test_unrelated_rule_does_not_use_var_reference_repr fails on master with:

state = UnrelatedCall()
a_0 = state.add_a(a=0)
state.unrelated(value=a_0) # should be value=0

I think the main use case for the singleton printers is addressed with VarReference, and any remaining effects are imo net-negative, though they can sometimes be more readable even if not strictly correct.

[Zac-HD]

I think here we actually do want

Suggested change

	values_0 = state.f(value=[])
	state.f(value=[[]])
	values_0 = state.f(value=[])
	state.f(value=[values_0])

because e.g. lists are mutable and so it's valuable to show the variable name which communicates the identity of the value.

I realize that this is Quite A Can Of Worms, what with not wanting values_0 = state.f(value=values_0); your motivating case in the changelog is spot-on but here and in the changed test I think the change is probably for the worse :/

[tybug]

agree that this case is worse than it used to be, but it's also not a correct transformation to make in general; see SourceSameAsTargetUnclearOrigin, where we would get state.f(value=[values_0]) even if the inner empty list came from st.just([]). And one could imagine more complicated cases of building up the inner list via some alternative st.composite where this would be more misleading.

I think we have to decide which of the two evils is worse...I believe the only case worsened by this is strategies involving bundles, which isn't great but is less common than rules involving bundles. The latter case collides relatively frequently thanks to shrinking normalizing to e.g. lots of zeros.

[Zac-HD]

What if we did self.__printer.singleton_pprinters.setdefault(id(result), printer), but only for objects which are not actually singletons? i.e. excluding None, True, False, small integers; we're still going to miss some cases but it seems closer to a strict improvement on the status quo.