Motivation: see "Other goodies" section at
https://github.com/leanprover/lean/wiki/Refactoring-structures
We had to add a new transparency mode: Instances at type_context.
In this mode, instances and reducible definitions are considered
transparent.
The new mode is used in the defeq_canonizer, code generator,
and sizeof lemma generation at inductive_compiler.
We also use the new mode in the unfold tactics.
@digama0 After this commit, your example will also produce a
non-destructive update.
```lean
structure test :=
(data1 : array nat 3)
(data2 : array nat 3)
(sz: nat)
def test.write (s : test) (i : fin 3) (v : nat) :=
{s with data1 := s^.data1^.write i v, data2 := s^.data2^.write i v}
set_option trace.array.update true
(fin.of_nat 1) 10 in
(a^.data1^.read (fin.of_nat 1), a^.data2^.read (fin.of_nat 2)) -- destructive write
```
