plugged a memleak

[Andy Wingo]

Hi all,

I just fixed a long-standing memory leak in Guile 2.0.  (I say that like
I'm proud or something, but of course I was the one the introduced it in
the first place!)

See c05805a4ea764dec5a0559edefcdfb9761191d07 in stable-2.0 for the
gnarly details.  The summary is that applicable smobs were being leaked,
because they were referenced in the values of weak-key tables.

This fixes Guile's compilation, where compiling eval.go would take half
a gig of memory.  It also fixes the web server, where
call-with-output-bytevector would leak a reference to the port and
bytevector.  Probably it fixes a number of other leaks.

Anyway, applied on both branches.

Happy hacking,

Andy
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