Add optimizations for case when result is equal

when cakeml is given
```
fun evaluated_case_elim x =
case x of
A a => A a
| B a b => B a b
| C a b c => C a b c;
```
it results in cakeml doing a heap allocation to deconstruct and reconstruct the result.
there should be an optimization that makes it rewritten to a form like
```
fun evaluated_case_elim x =
case x of
A a => x
| B a b => x
| C a b c => x;
```
and it should be done such that when #1106 is done it is
simplified into an identity function

The issue provides no file, test, or entry point. Start by locating CakeML's case-elimination transformation and reviewing the interaction described with #1106. Done means equivalent constructor-rebuilding cases avoid the unnecessary allocation and simplify to an identity function when #1106 is complete.

Written by the indexing model from the issue text.