implement _resolve_multibyte

- add unicode-derived 'Keystroke' class, which is yielded by the
generator method _resolve_multibyte, adding additional properties,
.is_sequence (bool), .code (int), and .name (str).
- detect encoding and initialize incrementaldecoder in __init__, used
by _resolve_multibyte.
- inherit curses key capability names, values, and create multibyte
sequence pattern lookup in __init__
- add _resolve_keycode method, returns key capability name for an
integer.
- add _resolve_multibyte(text) method, given a string of multibyte
input, 'text', returns instances of Keystroke class, which may be
checked for .is_sequence, .code, and .name.