Auto merge of #35712 - oli-obk:exclusive_range_patterns, r=nikomatsakis

Auto merge of #35712 - oli-obk:exclusive_range_patterns, r=nikomatsakis

exclusive range patterns

adds `..` patterns to the language under a feature gate (`exclusive_range_pattern`).

This allows turning

``` rust
match i {
    0...9 => {},
    10...19 => {},
    20...29 => {},
    _ => {}
}
```

into

``` rust
match i {
    0..10 => {},
    10..20 => {},
    20..30 => {},
    _ => {}
}
```