Check pantheon::String correctness for right-to-left languages

[bSchnepp]

Correct parsing of length and other properties for these languages is currently not done.

Specifically, some test cases need to be added to check that:
- The length of the right-to-left (or left-to-right) marker character is ignored for CharLength(), but counted for DataLength()
- That a single character is always counted as a single character, even if that character is modified by a subsequent letter.
- That the returned value of a given character is the byte at the location, per the specification for operator[].
- Mixing these with Latin script should work as intended.