Collect move errors before reporting

This commit changes the way move errors are reported when some value is
captured by a PatIdent. First, we collect all of the "cannot move out
of" errors before reporting them, and those errors with the same "move
source" are reported together. If the move is caused by a PatIdent (that
binds by value), we add a note indicating where it is and suggest the
user to put `ref` if they don't want the value to move. This makes the
"cannot move out of" error in match expression nicer (though the extra
note may not feel that helpful in other places :P). For example, with
the following code snippet,

```rust
enum Foo {
    Foo1(~u32, ~u32),
    Foo2(~u32),
    Foo3,
}

fn main() {
    let f = &Foo1(~1u32, ~2u32);
    match *f {
        Foo1(num1, num2) => (),
        Foo2(num) => (),
        Foo3 => ()
    }
}
```

Errors before the change:

```rust
test.rs:10:9: 10:25 error: cannot move out of dereference of `&`-pointer
test.rs:10         Foo1(num1, num2) => (),
                   ^~~~~~~~~~~~~~~~
test.rs:10:9: 10:25 error: cannot move out of dereference of `&`-pointer
test.rs:10         Foo1(num1, num2) => (),
                   ^~~~~~~~~~~~~~~~
test.rs:11:9: 11:18 error: cannot move out of dereference of `&`-pointer
test.rs:11         Foo2(num) => (),
                   ^~~~~~~~~
```

After:

```rust
test.rs:9:11: 9:13 error: cannot move out of dereference of `&`-pointer
test.rs:9     match *f {
                    ^~
test.rs:10:14: 10:18 note: attempting to move value to here (to prevent the move, use `ref num1` or `ref mut num1` to capture value by reference)
test.rs:10         Foo1(num1, num2) => (),
                        ^~~~
test.rs:10:20: 10:24 note: and here (use `ref num2` or `ref mut num2`)
test.rs:10         Foo1(num1, num2) => (),
                              ^~~~
test.rs:11:14: 11:17 note: and here (use `ref num` or `ref mut num`)
test.rs:11         Foo2(num) => (),
                        ^~~
```

Close #8064