Add exercise that checks sizes of data types

My textbook doesn't give a working example of how to get the size of the
'long' type, but this covers the others.

Planning on adding more exercises in this directory.
Add MIT license

Also made an update to the README
Move data_type_size_checker into solutions folder

I'm reorganizing the code samples to present them as solutions to
exercises. This one didn't seem to fit anywhere, but I'm still putting
it in the solutions directory to start with.
Add notes on data types
Add exercise of finding size of data types