strings_one.b is the second string implementation from class today,
a string consists of just one vector that contains the capacity, current
length, and all the characters.

Because increasing the size of a string results in a totally new string
object being created, every operation that could increase the size must
return the (new) string object and that returned value must overwrite
the variable that refers to the string.

stringstream_one.b is a slightly enlarged stringstream object as we had
by the end of the class.

From these two files you can see how easy it is to split a program into
multiple files. strings_one.b has an "export" statement that simply
lists the things that are to be made available to any program that
imports "strings_one", everything after that is automatic. You must
remember that if strings_one is modified, you need to prep strings_one
again and then re-prep stringstream_one again even if it has not been
changed. If you only modify stringstream_one, it is a waste of time to
re-prep strings_one.

strings_two.b implements a string as two vectors, one contains just the
characters, the other contains the capacity, current length, and a pointer
to the character vector. This means that when the string changes size,
only the character vector gets replaced. The three-item vector is updated
to point to the new character vector, so it does not need to be replaced.
I think the result is neater and easier to use, even if it is a little
less efficient.

stringstream_two.b is just stringstream_one.b adapted for this other style
of string implementation.