[plt-scheme] 3.99.0.3

The SVN trunk now contains version 3.99.0.3.

The changes have to do with namespace support in `scheme/base'. The
`make-namespace' procedure was renamed `make-empty-namespace', which is
a better description and avoids confusion with `make-namespace' from
`mzscheme'. The new `define-namespace-anchor' form and
`namespace-anchor->[empty-]namespace' procedures provide better support
for bridging the `module' and namespace worlds.

Chapter 18 of the guide now has a bit of text to describe the new
pieces. (I have to add a lot more to that chapter.)

Matthew