Changes to SrPersist code

["Paul Steckler" <steck@ccs.neu.edu>]

I've exp-tagged a bunch of changes to the SrPersist code.

Changes:

 - all ODBC procedures are available in Scheme regardless of the ODBC 
    version used to compile the extension.
 - if a procedure is not defined in the ODBC version used when 
    compiling, calling it from Scheme raises a not-implemented 
    exception.
 - the invoke-n.m.ss files used to load the extension have been 
    collapsed into one file.  To load the extension, you just run
    
	(require (lib "srpersist.ss" "srpersist"))

 - buffers, indicators, lengths, and row-status values may be explicitly
    freed.  Yes, this approach opens a safety hole, and you may hurl 
    brickbats: but it was always possible to overflow a buffer, so 
    safety was already compromised.  Programs that use SrPersist can now

    avoid memory leaks.  In particular, functional layers like SchemeQL 
    can manage memory for the programmer who (sensibly) doesn't want to 
    use ODBC directly.
 - the usual miscellaneous small bug fixes.

I'm told a shiny new version of SchemeQL to match this version of
SrPersist is due soon.

-- Paul