Fix str() bug for tuple values (#33)

Replaced the use of old-style formatting strings in __str__ methods.

At first, that seems like a dated but harmless practice, but unfortunately

While dated and seemingly a harmless practice, OSlash actually uses it
in an incorrect way.

If a program using OSlash deals with types other than strings, then the
__str__ methods for classes like `Left`, `Right`, and `Either`, OSlash
directly writes:

    return '%s' % value

where `value` is an arbitrary type.

Unfortunately, '%s' % value is only guaranteed to work if the value
is a string, since Python will not call `str()` on it.

(Well, more precisely, `'%s' % value` is guaranteed to *fail* if the
value happens to be a tuple of a certain kind, since Python will try
to unpack the tuple into the string formatting operation)

The upshot is that such programs will get TypeErrors.  Instead, use
Python 3.8+ f-strings to guarantee that the value is converted to a
string.

Co-authored-by: Dag Brattli <[email protected]>

Author: cfs-pure