[R6RS] Core/library split

[Anton van Straaten]

I've committed an update of core-library.txt, library-contents.html, 
library-index.html, and library-contents.scm.

Changes include:

* Deleted the r6rs-small library, since the base library seems to 
obviate it now.
* Corrected the inconsistency regarding generic arithmetic, which 
remains in r6rs-base but is no longer mentioned as a separate library.
* Created a mutable-list library for set-car! and set-cdr!.

Other than that, I've made a conservative set of changes based on what 
Will and Mike agreed on, biased towards keeping disputed items where 
they already were.  Changes include:

* Removed non-library read syntax items
* Moved the condition and exception items which Mike specified, out of 
the base.  (Will would like to remove more.)
* added error, violation & contract-violation to base
* moved call/cc & dynamic-wind to a 'control' library

'R6rs-base' is not as large as Will has suggested (e.g. no bytes, 
port-i/o, or records).  Accordingly, I've retained both r6rs and 
r6rs-big, but their composition has changed, as listed in 
core-library.txt.  The 'r6rs' library has grown, and somewhat resembles 
what Will wants in r6rs-base.  r6rs-big is intended as the library which 
includes (almost) everything defined in R6RS, but it is still missing 
some libraries.

I've also added the detailed listing of contents for r6rs and r6rs-big 
to library-contents.html.

I haven't yet moved syntax-rules into the base.

Anton