Precise range exhaustiveness check for match on integer patterns

In theory the compiler should be able to detect that the following range is exhaustive and not error about non-exaustive match:

``` rust
fn wont_compile(x : u8) { 
    match (x) {
        0x00 ... 0xff => { }
    }
}
```

Proposed (as a side-portion) in #880

Discussed in https://github.com/rust-lang/rust/issues/12483 and https://github.com/rust-lang/rust/issues/32381
