This PR refactors match compilation, to handle “side-effect free”
patterns (`.var`, `.inaccessible`, `.as`) eagerly and for each
alternative separately. The idea is that there should be less interplay
between different alternatives, and prepares the ground for #11105.
This may cause some corner case match statements to compiler or fail
compile that behaved differently before. For example, it can now use a
sparse case where previously was using a full case, and pattern
completeness may not be clear to lean now. On the other hand, using a
sparse case can mean that match statements mixing matching in indicies
with matching on the indexed datatype can work.
This PR lets the match compilation procedure use sparse case analysis
when the patterns only match on some but not all constructors of an
inductive type. This way, less code is produce. Before, code handling
each of the other cases was then optimized and commoned-up by later
compilation pipeline, but that is wasteful to do.
In some cases this will prevent Lean from noticing that a match
statement is complete
because it performs less case-splitting for the unreachable case. In
this case, give explicit
patterns to perform the deeper split with `by contradiction` as the
right-hand side.
At least temporarily, there is also the option to disable this behaviour
with
```
set_option backwards.match.sparseCases false
```
