Default value adt fixes

[lorchrob]

Two bug fixes

First, a straightforward bug fix in default_value in lustreDesugarADTs.ml

Second ---
Say we have t = C(0), and t' = C(0), but t and t' have different junk fields. Say we have set s. When determining t in s or t' in s, clearly only the non-junk fields should be considered. But, our encoding is a mapping from all the fields to a boolean value. So, in my opinion, the cleanest thing to do is to canonicalize junk fields to a default value -- when compiling t in s for ADT expression t, we essentially compute s[t_tag][canonicalize(t_field0)][canonicalize(t_field1)]..., where canonicalize is just an ITE: if constructor_selected then t_fieldn else default_value. Then clearly, we have to also update set insertion analogously.
To be clear, what we were doing before this PR is s[t_tag][t_field0][t_field1]..., the same thing but without the canonicalize ITE