Warning on u8 -> char casts; warning: can't cast this type, #[warn(const_err)]

This is a new warning in recent nightlies, for u8 -> char casts.

``` rust
src/bin.rs:71:13: 71:25 warning: can't cast this type, #[warn(const_err)] on by default
src/bin.rs:71     if c >= 33u8 as char && c < 128u8 as char {
                          ^~~~~~~~~~~~
```
