srfi-55 support added

[Rob Browning]

The development tree now supports a quick implementation of SRFI-55
(require-extension) in boot-9.scm.  See
http://srfi.schemers.org/srfi-55/srfi-55.html.

require-extension is available without loading any modules, and so it
provides a portable way to load SRFIs, i.e. (require-extension (srfi 1
13)).

Note that the current implementation expands into a suitable set of
use-modules statements, and so it can only be called from the
top-level.  The SRFI specification seems somewhat vague on the point,
and so it's not entirely clear if such a restriction is or isn't OK.

Simple tests are in test-suite/standalone/test-require-extension.

I also just realized that I forgot to add appropriate info
documentation.  I'll try to make some time to do that, but if anyone
else is motivated, go right ahead.

Thanks
-- 
Rob Browning
rlb @defaultvalue.org and @debian.org; previously @cs.utexas.edu
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