[plt-scheme] 350.3

MzScheme and MrEd are now version 350.3 in the SVN repository trunk.

Changes:

 * `hash-table-get' now accepts a non-procedure third argument as a
    default result (for the case that no value is found in the hash
    table for the key). If a procedure is supplied as the third
    argument, it is still called as a failure thunk.

    Of course, if the default value is supposed to be a procedure, it
    must be wrapped in a thunk. That's a little awkward, but I prefer
    it to breaking backward compatibility, defining multiple
    `hash-table-get' functions, or continuing to require that simple
    constants are thunked.

 * Improved performance of the 3m GC.


Matthew