Preserve name of default branch in `core::Term::ConstMatch`

[Kmeakin]

At the moment, we throw away the name of the binder when elaborating pattern matches to ConstMatch with a default branch

eg

match x {
    0 => 1,
    y => y,
}

elaborates to

match x {
    0 => 1,
    _ => _,
}

This patch preserves the name of the binding, making the elaborated core more readable

[brendanzab]

This is great, thanks!