lib: combine cat_str and vscat implementations.

The vscat function is white-box copy of cat_str, with just the
iteration over the inputs done differently, and without the
support for separators that are characters instead of strings
(which was added to cat_str after vscat was forked.

In this patch, the common logic underlying both functions is
factored out into a small ad-hoc "struct cat_str" object which
maintains the state and provides the operations to measure the
pieces of the string, allocate the space, copy the pieces
together and produce the resulting object.

The motivation here isn't just to reduce duplication.  I would
like a more efficient function for catenating strings which
takes a "struct args *", not requiring a list to be consed up.

* lib.c (struct cat_str): New struct type.
(cat_str_init, cat_str_measure, cat_str_alloc, cat_str_append,
cat_str_get): New static functions.
(cat_str, vscat): Considerably shorten by using the above
functions.

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