Feature request: match_arm_align_threshold

[cehteh]

Since we already have:

enum_discrim_align_threshold
struct_field_align_threshold

It would be nice to have a similar formatter for simple/one-line match arms that lines up the '=>' like:

match foo {
    Some(LongEntityName(name))  => 1,
    Some(Short(thing))          => 2,
    Some(_)                     => 3,
    None                        => 4,
}

[calebcartwright]

Closing as a duplicate of #894 (though I'll add that i think your proposal is probably as good a solution as any 👍)