* Enforce the U+0000 to U+10FFFF range in `count` and throw an error if
the given codepoint exceeds the range (encoding U+0000 won't work of
course, but this is Nix's fault…).
* Check if the produced bytes are well formed and output an error if
not. This indicates that the codepoint can't be encoded as UTF-8, like
U+D800 which is reserved for UTF-16.
Change-Id: I18336e527484580f28cbfe784d51718ee15c5477