Wrote all remaining Unicode input and output functions.

Wrote all remaining Unicode input and output functions.
Updated gestalt to support Unicode.
Split off functions to convert from one character set to another and put them in charset.c.

glk_put_char_uni(), glk_put_string_uni(), glk_put_buffer_uni(),
glk_put_char_stream_uni(), glk_put_string_stream_uni(),
glk_put_buffer_stream_uni(), glk_get_char_stream_uni(),
glk_get_buffer_stream_uni(), glk_get_line_stream_uni()