Rename unicode_t into char32_t

The type char32_t represents a unicode codepoint
encoded in UTF-32 (same as wchar_t on Linux).
char32_t is the type used in the upcoming C11
standard.

The file "C-kern/api/string/unicode.h" which defines char32_t
is also moved into directory "C-kern/api/context/" and included
from the "konfig.h" file.
Integrate textpos_t into utf8reader_t

Also rename io/reader/text subdirectory into io/reader/util.
Implement remove_,load_,save_file

The load and save functions are moved into their
own module fileutil.c.

The implementation of wbuffer_t is changed to support
other (currently not implemented) memory allocation strategies.

Introduced and explained parameter type /*ret*/ in
'coding-conventions/parameter_type.txt'.