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.
