Often, an implementation of VariableMap can return references to
internal strings, and Cow::Borrow() allows that without making any
copies.
We still want to allow VariableMap implementations to return owned
strings in case they have to manufacture variable values on demand. The
Cow::Owned() variant does exactly that.
Switch the internal VariableMap implementations over to Cows. It turns
out they can simply store references to substrings of the input test,
completely avoiding string allocation for script-defined variables.
The -c flag to 'cton-util filecheck' will now print out a description of how
the directives are matching the input.
This explanation is also printed when a match fails.
