core: Remove an implicit panic from Formatter::pad

core: Remove an implicit panic from Formatter::pad

The expression `&s[..i]` in general can panic if `i` is out of bounds or not on
a character boundary for a string, and this caused the codegen for
`Formatter::pad` to be a bit larger than it otherwise needed to be. This commit
replaces this with `s.get(..i).unwrap_or(&s)` which while having different
behavior if `i` is out of bounds has a much smaller code footprint and otherwise
avoids the need for `unsafe` code.