New version of new GC scheme

[Mikael Djurfeldt <mdj at mdj dot nada dot kth dot se>]

I just committed a new version of the GC code.

News are:

1. The code is enabled by default (so you get this code unless you
   edit away GUILE_NEW_GC_SCHEME in __scm.h).

2. The environment variables have new names:

   GUILE_INIT_SEGMENT_SIZE_1
   GUILE_GC_TRIGGER_1
   GUILE_INIT_SEGMENT_SIZE_2
   GUILE_GC_TRIGGER_2

3. If you specify a negative integer for the triggers, this is a
   fraction in percent of total heap size.  E.g., to get GC trigger on
   a fourth of the heap, set gc trigger to -25.  (Positive values
   mean absolute value in number of cells.)

4. There's a new environment variable to specify
   GUILE_MAX_SEGMENT_SIZE

I've also cleaned up the code a bit.  It was partially quite terrible
earlier.  I hope I'll have some time to document it a bit better
tomorrow.