Merged in pqR's 03-fast-base, with minor modifications. This provided
a shortcut for finding the value of base variables when these are in
the global cache.


git-svn-id: https://svn.r-project.org/R/trunk@63468 00db46b3-68df-0310-9c12-caf00c1e9a41
Added write barrier testing support for fast-base changes.


git-svn-id: https://svn.r-project.org/R/trunk@63469 00db46b3-68df-0310-9c12-caf00c1e9a41