[plt-dev] #lang at-exp

If you want to enable @-expressions in a module that is written in,
say, `scheme/base', the old way was to start the file like this:

  #reader scribble/reader
  #lang scheme/base

The new way is like this:

  #lang at-exp scheme/base

In general, `at-exp' is a kind of language constructor that installs
"@" support (through the readtable) before chaining to a base language.

Eventually, DrScheme and other tools will work better on files that
start "#lang". The old "#reader" way works only because an S-expression
reader happens to be active before "#lang".