utf8: Fix encoding of U+FFFE and U+FFFF #107

Job	Run time
windows (shared)	2m 5s
linter	1m 0s
windows (static)	2m 31s
posix (linux, clang, shared)	29s
posix (linux, clang, shared, ASan)	27s
posix (linux, clang, static)	24s
posix (linux, clang, static, ASan)	22s
posix (linux, gcc, shared)	11s
posix (linux, gcc, shared, ASan)	24s
posix (linux, gcc, static)	21s
posix (linux, gcc, static, ASan)	50s
posix (macos, clang, shared)	1m 9s
posix (macos, clang, static)	21s
posix (macos, clang, static, ASan)	2m 5s
	12m 39s

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