Improve formatting of pattern alternatives

I found no mention in the spec of how to format pattern alternatives like A | B | C => expr, so I have to assume that what rustfmt does there is some ad-hoc decision made during implementation. I think that formatting should be given some thought. I am not sure where to start with that, so I figured I'd open an issue. ;)

Some of the problems that we currently have:

• With long lists of patterns (that do not fit on a single line), we end up with code like this:

Pat1
| Pat2
| Pat3
| Pat4 => ...

Notice how the patterns start at different vertical positions, destroying symmetry. This is unlike how every other "list-like thing" is formatted, which all are properly vertically aligned. Also see https://github.com/rust-lang/rustfmt/issues/3973.

• With long matches, rustfmt can end up generating code like this:

        match ty.kind {
            // Types without identity.
            ty::Bool
            | ty::Char
            | ty::Int(_)
            | ty::Uint(_)
            | ty::Float(_)
            | ty::Str
            | ty::Array(_, _)
            | ty::Slice(_)
            | ty::RawPtr(_)
            | ty::Ref(_, _, _)
            | ty::FnPtr(_)
            | ty::Never
            | ty::Tuple(_)
            | ty::Dynamic(_, _) => self.pretty_print_type(ty),

            // Placeholders (all printed as `_` to uniformize them).
            ty::Param(_) | ty::Bound(..) | ty::Placeholder(_) | ty::Infer(_) | ty::Error => {
                write!(self, "_")?;
                Ok(self)
            }

Notice how the two lists of alternatives are formatted very differently. This should be done in a more consistent way. Just like https://github.com/rust-lang/rustfmt/issues/3995 is about formatting the match arms themselves consistent across a match, I think rustfmt should also format lists of pattern alternatives consistent across a match.

Things get even more complicated when considering match guards... but one step after the other, I'd say. ;)

Start by reviewing the issue's examples of multiline pattern alternatives and the related issues 3973 and 3995. The payload names no files, tests, or entry points, so locating the formatter logic and agreeing on the desired formatting rules would be the first step. Done means pattern alternatives are formatted consistently, including across match arms, with match guards considered as scope allows.