std::str: Remove functions count_chars, count_bytes

[bluss]

These are very easy to replace with methods on string slices, basically
.char_len() and .len().

These are the replacement implementations I did to clean these
functions up, but seeing this I propose removal:

/// ...
pub fn count_chars(s: &str, begin: uint, end: uint) -> uint {
// .slice() checks the char boundaries
s.slice(begin, end).char_len()
}

/// Counts the number of bytes taken by the first n chars in s
/// starting from byte index begin.
///
/// Fails if there are less than n chars past begin
pub fn count_bytes<'b>(s: &'b str, begin: uint, n: uint) -> uint {
s.slice_from(begin).slice_chars(0, n).len()
}