[plt-scheme] shadowing initial imports (3.99.0.20)

Version 3.99.0.20 implements Jos's suggestion:

 http://list.cs.brown.edu/pipermail/plt-scheme/2008-February/023282.html


This change means, for example, that you can write

  #lang scheme
  (require srfi/1)

in which case the body of the module uses `map', `third', and several
other names from `srfi/1' instead of from `scheme'.

This shadowing rule applies only to the initial import (a.k.a.
"language" of the module). For example,

  #lang scheme
  (require scheme srfi/1)

still reports a syntax error, since `scheme' is (also) `require'd in
the module body, and `scheme' and `srfi/1' provide many of the same
names with different bindings.


Matthew