More cleanup of the bytecode stuff; in particular, add a `MB_' prefix

Estimated hours taken: 2

More cleanup of the bytecode stuff; in particular, add a `MB_' prefix
to most names.

bytecode/*:
	- Prefix all names defined in header files with `MB_'
	  (except for `TRUE', `FALSE', `DEBUGGING', and `XXXdebug').
	- Add macros `MB_new', `MB_new_array', and `MB_resize_array',
	  and use those instead of using `MB_malloc' and `MB_resize'
	  (formerly `mem_malloc' and `mem_resize') or malloc() and realloc()
	  directly.  Uncomment the definition of MB_strdup() now that it
	  calls MB_malloc().
	- Delete the definitions of `uchar', `uint', `ushort', `ulong'
	  (just spell them out, its clearer and more portable that way).
	- Fix the indentation in a few places I missed on my previous pass.
	- Add a `README' file.

bytecode/README

@@ -0,0 +1,9 @@
+This directory holds the source code for the Mercury bytecode
+utilities. `mdis' is the Mercury bytecode disassembler.
+`mbi' is the Mercury bytecode interpreter.
+(XXX should it be a stand-alone program, or should it just be a library?)
+
+All exported symbols defined in modules in this directory should be prefixed
+with `MB_', for "Mercury Bytecode", to avoid clashes with names defined in
+other packages.
+