Print proc_macro spans as a half-open range

A span covering a single byte, such as for an operator `+` token, should
print as e.g. `80..81` rather than `80...81`. The lo end of the range is
inclusive and the hi end is exclusive.

src/libproc_macro/lib.rs

@@ -338,7 +338,7 @@ impl Span {
 #[unstable(feature = "proc_macro", issue = "38356")]
 impl fmt::Debug for Span {
  fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
- write!(f, "{:?} bytes({}...{})",
+ write!(f, "{:?} bytes({}..{})",
  self.0.ctxt(),
  self.0.lo().0,
  self.0.hi().0)