This example demonstrates how to encode string
features efficiently by creating a more compactly encoded
bit-string from StringCharFeatures.
For instance, when working with the DNA alphabet {A,T,G,C}
using 1 char = 1 byte per symbol would be wasteful, as we
can encode 4 symbols using 2 bits only.
Here, this is done in junks of 64bit (ulong).
