bug#17502: R6RS `library' form must have exports before imports

[Taylan Ulrich Bayirli/Kammer]

The R6RS `library' form requires the (export ...) list to appear
before the (import ...) list.  I'm not sure if this itself is a bug;
R6RS consistently uses that order, doesn't mention the order to be
irrelevant, and seems to me to imply that it's indeed mandatory.  I
didn't check for what other R6RS implementations do though.

Here's a corresponding documentation patch against master/3be43fb:

0001-R6RS-library-documentation-fix.patch
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