Detect >= to => typo in comparisons

[estebank]

When encountering code like if 42 => 2 {}, we currently emit

error: expected `{`, found `=>`
 --> src/main.rs:2:10
  |
2 |    if 42 => 2 {}
  |          ^^ expected `{`
  |
note: the `if` expression is missing a block after this condition
 --> src/main.rs:2:7
  |
2 |    if 42 => 2 {}
  |       ^^

But it should be closer to

error: expected `{`, found `=>`
 --> src/main.rs:2:10
  |
2 |    if 42 => 2 {}
  |          ^^ expected `{`
  |
note: you likely meant to write a "greater than or equal to" comparison
 --> src/main.rs:2:7
  |
2 -    if 42 => 2 {}
2 +    if 42 >= 2 {}
  |

[sjwang05]

Hi! I'd like to try working on this issue if nobody else is currently.

@rustbot claim