Exhaustive integer pattern matching

[varkor]

Extend Rust's pattern matching exhaustiveness checks to cover the integer types: u8, u16, u32, u64, u128, usize, i8, i16, i32, i64, i128, isize and char.

fn matcher_full(x: u8) {
  match x { // ok
    0 .. 32 => { /* ... */ }
    32 => { /* ... */ }
    33 ..= 255 => { /* ... */ }
  }
}

fn matcher_incomplete(x: u8) {
  match x { //~ ERROR: non-exhaustive patterns: `32u8...255u8` not covered
    0 .. 32 => { /* ... */ }
  }
}

This feature has already been implemented behind the feature flag exhaustive_integer_patterns, so this RFC is viewed as a motion to stabilise the feature. See rust-lang/rust#50912 (comment) for the previous motion to (unstably) approve this feature.