slib rev2 -1+, <?, ...

[Kevin Ryde]

        * slib.scm (-1+, <?, <=?, =?, >?, >=?): Define as aliases for 1-, <,
        <=, =, >, >= respectively, required by slib 'rev2-procedures but no
        longer in the guile core.

I hope this is the right way to go about this.  Ie. if rev2-procedures
is in the features list then guile should be providing these defines.

I guess the alternative would be to drop it from the features list and
let slib make the necessary definitions.  That'd lose the guile core
defines of other things like append! and substring-fill! though.

In the 1.6 branch -1+ still exists, but the others don't, so this
would be a proposal to add them there similarly too.

slib.scm.ref2.diff
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