[R6RS] Export level of `set!'

[R. Kent Dybvig]

> I'm working on addressing tickets #159 and #164.  Just to make sure:
> `set!' is exported from (r6rs base) at levels 0 and 1, right?  (Level 0
> for the regular `set!', and level 1 for the occurrence in
> `identifier-syntax'.)

Yes.  Good catch.

Kent