Update match checking.

Update match checking.

fn is_useful , more skeletons

Specify a lifetime on pattern references

impl PatStack

fill impl Matrix

PatStack::pop_head_constructor

Index-based approach

struct PatCtxt

fields construction fn Fields::wildcards

split wildcard

fn Constructor::is_covered_by_any(..)

fn Matrix::specialize_constructor(..)

impl Usefulness

Initial work on witness construction

Reorganize files

Replace match checking diagnostic

Handle types of expanded patterns

unit match checking go brrr