TextSegment: read methods

Just like the already existing read methods, which work globally on the
text, there are multiple ways of reading from a single segment. It is
possible to read a single character, a string denoted by an index and an
offset and a string denoted by a start index and an end index.
In addition, there is also a way to get the entire string, which will be
useful for copying segments inbetween a specified range.