auto merge of #16885 : wickerwaka/rust/fmt-center, r=alexcrichton

auto merge of #16885 : wickerwaka/rust/fmt-center, r=alexcrichton

Use '^' to specify center alignment in format strings.

```
fmt!( "[{:^5s}]", "Hi" ) -> "[ Hi  ]"
fmt!( "[{:^5s}]", "H" )  -> "[  H  ]"
fmt!( "[{:^5d}]", 1i )   -> "[  1  ]"
fmt!( "[{:^5d}]", -1i )  -> "[ -1  ]"
fmt!( "[{:^6d}]", 1i )   -> "[  1   ]"
fmt!( "[{:^6d}]", -1i )  -> "[  -1  ]"
```

If the padding is odd then the padding on the right will be one
character longer than the padding on the left.