Clean up the other Slice*Inclusive impls for str

A previous PR fixed one method that was legitimately buggy;
this cleans up the rest to be less diverse, mirroring the
corresponding impls on [T] to the greatest extent possible
without introducing any unnecessary UTF-8 boundary checks at 0.

Author: ExpHP

src/libcore/str/mod.rs

@@ -2035,19 +2035,13 @@ mod traits {
         type Output = str;
         #[inline]
         fn get(self, slice: &str) -> Option<&Self::Output> {
-            if let Some(end) = self.end.checked_add(1) {
-                (self.start..end).get(slice)
-            } else {
-                None
-            }
+            if self.end == usize::max_value() { None }
+            else { (self.start..self.end+1).get(slice) }
         }
         #[inline]
         fn get_mut(self, slice: &mut str) -> Option<&mut Self::Output> {
-            if let Some(end) = self.end.checked_add(1) {
-                (self.start..end).get_mut(slice)
-            } else {
-                None
-            }
+            if self.end == usize::max_value() { None }
+            else { (self.start..self.end+1).get_mut(slice) }
         }
         #[inline]
         unsafe fn get_unchecked(self, slice: &str) -> &Self::Output {
@@ -2076,29 +2070,21 @@ mod traits {
         type Output = str;
         #[inline]
         fn get(self, slice: &str) -> Option<&Self::Output> {
-            if self.end < usize::max_value() && slice.is_char_boundary(self.end + 1) {
-                Some(unsafe { self.get_unchecked(slice) })
-            } else {
-                None
-            }
+            if self.end == usize::max_value() { None }
+            else { (..self.end+1).get(slice) }
         }
         #[inline]
         fn get_mut(self, slice: &mut str) -> Option<&mut Self::Output> {
-            if self.end < usize::max_value() && slice.is_char_boundary(self.end + 1) {
-                Some(unsafe { self.get_unchecked_mut(slice) })
-            } else {
-                None
-            }
+            if self.end == usize::max_value() { None }
+            else { (..self.end+1).get_mut(slice) }
         }
         #[inline]
         unsafe fn get_unchecked(self, slice: &str) -> &Self::Output {
-            let ptr = slice.as_ptr();
-            super::from_utf8_unchecked(slice::from_raw_parts(ptr, self.end + 1))
+            (..self.end+1).get_unchecked(slice)
         }
         #[inline]
         unsafe fn get_unchecked_mut(self, slice: &mut str) -> &mut Self::Output {
-            let ptr = slice.as_ptr();
-            super::from_utf8_unchecked_mut(slice::from_raw_parts_mut(ptr as *mut u8, self.end + 1))
+            (..self.end+1).get_unchecked_mut(slice)
         }
         #[inline]
         fn index(self, slice: &str) -> &Self::Output {