utility: imported duct::signed_cast() and duct::unsigned_cast().
Widget: added input-control mode.
String: added char8 alias.
Moved textual stuff into namespace txt; added text handling classes.

tty::Terminal: added points param to put_sequence().
tty: moved EncUtils, UTF8Block, and Sequence to namespace txt.

txt: added classes Tree, Node, and Cursor.

test/txt: added simple operation test.
ui: added Field widget.

test/ui/packing: added Field widgets.
ui::Widget::Base: only render debug frame when NDEBUG.