Remove Gtk junk, since I've never used it
Add missing const in recursion_preamble
Constify global state when passed through recursion functions

(Haven't fixed the function signature in sequence.c, since
that file is very broken right now.)
Make sequence->recurse multithreaded

Basically, it spawns a maximum of 20 threads during the first
two levels of recursion.
Remove access to global state from subthreads

Subthreads now should only be dealing with their own private memory,
which will only be accessed by one other thread (the main one), and
only after they have died.

A consequence of this is that the max-length target now does not have
access to the actual object of maximum length. This will need to be
fixed.

But for now, we have a massive speedup.
Removed unused _blob structure from ramsey/sequence.c
Make coloring->recurse multithreaded

I will need to do some refactoring before merging, since most
of the new code I was able to copy-and-paste directly from the
sequence code.
Make recursion seed part of global state
Unify thread code from coloring and sequence into recurse.c
Stop redundantly passing the ramsey_t to recursion functions
Add clone() method to data collectors
Add clone method for global data
Add absorb() method to data collectors
Implement proper threading for sequence and coloring

Well, not quite "proper" - I still need to document the new code,
and am still using a fixed thread pool of size 10, and am still
spawning threads at depth 3 instead of something smarter.

But OTOH, the full program state is copied into each thread, and
merged back completely after the threads have died (so there is
no data sharing). I have also moved the thread management into
recurse.c, so that these can be stubbed out for non-threaded versions
without maintaining too much of a crazy branch.
Fix segfault when cloning iters-per-length
Reset data dumps when cloning for subthreads