Pre 1.9.2 bugfixes

[Mike Gran]

Hi-

I hope that master is closer to building cleanly today w.r.t. strings.

- Added a cast in SCM_MAKE_CHAR.  If that doesn't work, then maybe
SCM_MAKE_CHAR should become an inline function.
- Removed some signed/unsigned comparisons and conversions
- Applied Greg Troxel's patch to cast all input to tolower to int.
  (Crap.  I should have committed this under your name.  Sorry.)
- Added a scheme data type to mimic a uniconv data type, so that
<uniconv.h> doesn't appear in the Guile public headers
- removed calls to the local-independent uc_tolower for now.

Hope this helps,

-Mike