[R6RS] Export level of `set!'

[Michael Sperber]

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'.)

-- 
Cheers =8-} Mike
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