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.
It was being stored in the environment before. This was very hackish,
and it was producing a series of unnecessary environment updates, and
thread local caches invalidations.
The new test tests/lean/run/heap.lean is 5x-6x faster after this commit.
