Faster compilation speed

[Robert Dewar]

<        GCC did not switch from obstacks to garbage collection because of
any inherent love for garbage collection.  Using garbage collection
instead of obstacks was the most efficient way to support other features
which were added to GCC 3.0.
>

It also removes a pernicious variety of bug that often caused nasty memory
corruption in earlier versions of GCC. Our experience with the back end of
GCC (from the point of view of GNAT) is that code generation errors have
been a much more serious problem than time and space requirements.