Make panic message less confusing

The panic message when slicing a string with a negative length range (eg
`"abcdef"[4..3]`) is confusing: it gives the condition that failed to hold,
whilst all the other panic messages give the condition that did hold.

Before:
begin <= end (4 <= 3) when slicing `abcdef`

After:
begin > end (4 > 3) when slicing `abcdef`

Author: Kmeakin

library/alloctests/tests/str.rs

@@ -612,14 +612,14 @@ mod slice_index {
             data: "abcdef";
             good: data[4..4] == "";
             bad: data[4..3];
-            message: "begin <= end (4 <= 3)";
+            message: "begin > end (4 > 3)";
         }
 
         in mod rangeinclusive_neg_width {
             data: "abcdef";
             good: data[4..=3] == "";
             bad: data[4..=2];
-            message: "begin <= end (4 <= 3)";
+            message: "begin > end (4 > 3)";
         }
     }
 

library/core/src/str/mod.rs

@@ -99,7 +99,7 @@ fn slice_error_fail_rt(s: &str, begin: usize, end: usize) -> ! {
 
     // 3. range is backwards.
     if begin > end {
-        panic!("begin <= end ({begin} <= {end}) when slicing `{s_trunc}`{ellipsis}")
+        panic!("begin > end ({begin} > {end}) when slicing `{s_trunc}`{ellipsis}")
     }
 
     // 4. begin is inside a character.