[0/4] RFC: refactor partial symbol tables

[Tom Tromey <tromey at redhat dot com>]

For the past few months I have been working on a project to speed up
gdb startup.  In particular, I've been looking at way to speed up symbol
table reading.

This patch series is the first result of that work.  It changes gdb so
that partial symbols and partial symbol tables are hidden from the core
of gdb.  After this series, a given symbol reader can decide for itself
whether to use psymtabs or some other approach.

I have not included the actual speedups in this series.  I will send
those separately; I want to see first whether they will be acceptable
for Fedora, among other things.

The infrastructure provided by this series makes it very easy to
experiment with different approaches.  I've written 2 or 2.5 versions of
the indexing code, plus implemented psymtab reading in the background.

I am not going to commit this until after Keith's work has gone in, as
there are some conflicts, and he has done enough rebasing.

This series was built and regtested on x86-64 (compile farm).  I did not
individually check each patch, though I think they are independent.

Tom