String::Tagged implements an object class, instances of which store a (mutable)
string buffer that supports tags. A tag is a name/value pair that applies to
some non-empty extent of the underlying string.

The types of tag names ought to be strings, or at least values that are
well-behaved as strings, as the names will often be used as the keys in hashes
or applied to the eq operator.

The types of tag values are not restricted - any scalar will do. This could be a
simple integer or string, ARRAY or HASH reference, or even a CODE reference
containing an event handler of some kind.

Tags may be arbitrarily overlapped. Any given offset within the string has in
effect, a set of uniquely named tags. Tags of different names are independent.
For tags of the same name, only the latest, shortest tag takes effect.