Profiling showed that when loading a large .gfo file, shareString was responsible for
15-18% of the CPU time, and a lot of the allocation. Since we already use ByteStrings for
reading the source files, shareString mostly has the effect of creating lots
of small ByteStrings instead of one large one. Since the plain size of the .gfo is seldom
a problem (unlike when it was read as a String), it is ok to keep the whole file
as one ByteString in RAM, and have all tokens point into that.
Profiling after the change showed 15-20% reduction in CPU time and in total allocation.
