[plt-scheme] 299.406

MzScheme and MrEd are now version 299.406 in the SVN repository trunk.

The main changes are in mzc. If you use mzc to compile Scheme code to
native code, please check that mzc works for you and that the resulting
code performs about as well as before. (mzc at least passes the full
MzScheme test suite --- for the first time in the v299.x series.)

MzScheme has a new core form: `#%variable-reference'. This form obtains
the location of a top-level or module-bound variable; the location is
opaque to Scheme code, but useful to low-level tools such as mzc. I
expect that it will appear only in code generated by low-level tools.

Matthew