Give a better error message for `=>` in a return type

[jyn514]

Spot the difference (playground):

fn f() -> usize {
    0
}

fn g() => usize {
    0
}

error: expected `;` or `{`, found `=>`
 --> src/lib.rs:5:8
  |
5 | fn g() => usize {
  |        ^^ expected `;` or `{`

Notice how this gives a hard parse error and doesn't even mention return types. Instead, it should suggest g() -> usize.

[mibac138]

@rustbot claim