[R6RS] library name ambiguities

[R. Kent Dybvig]

I believe adding (library <library reference>) as an additional
<import spec>, and favoring the keyword interpretation of "library",
"and", "or", etc., solves the ambiguity problem in a minimal way.

For (import (except (and (1)))), this would force the first parse
described in

  http://lists.r6rs.org/pipermail/r6rs-discuss/2007-June/002554.html

i.e., an import from a library (and) with version 1.  To get the
second parse, one could write (import (library (except (and (1))))).

Of course, to use "library" as a library name, one could write

  (import (library (library)))

The library keyword would not often be needed, so it would not add
any verbosity to most import forms.

We might be able to come up with something better, but this fits with
our plan to making only easy changes.

Kent