bug#38236: R7RS reader does not support datum labels

[Andy Wingo]

R7RS defines a lexical feature called "datum labels"; see section 2.4 in
the report.  An example would be:

   #0=(a b c . #0#)

Guile's reader doesn't support this feature and it's not clear if we
should, in general.  Note, datum literals appear to be incompatible with
array literals.  But we could of course use the R7RS layer as a place to
experiment with a `read' implemented in Scheme.