Use correct and efficient boundary checks

[lo48576]

• Assertion for the index given for InlineString::remove() seems not correct (at least it is inconsistent with String::remove()).
  • It should not accept idx argument which is equal to self.len().
  • The first commit fixes this.
• Use efficient boundary check instead of traversing each character in a string.
  • Previous implementation uses self.char_indices().foo() for boundary check, but it is O(n) operation.
  • str type has quite efficient (O(1)) implementation.
  • The second commit improve them to use methods of str instead of iterators.