Make utf8_codecvt::out return partial for trailing surrogates

[Flamefire]

When a high surrogate is encountered there must be something following. If it is not, the input is incomplete, i.e. "partial".

See also https://github.com/boostorg/nowide/commit/14675cd82290fae97da7bfb1cd3aa001679c6850

Fixes #50