utf8: Fix encoding of U+FFFE and U+FFFF #108

Job	Run time
linter	48s
windows (shared)	1m 18s
windows (static)	1m 7s
posix (linux, clang, shared)	27s
posix (linux, clang, shared, ASan)	33s
posix (linux, clang, static)	30s
posix (linux, clang, static, ASan)	26s
posix (linux, gcc, shared)	14s
posix (linux, gcc, shared, ASan)	27s
posix (linux, gcc, static)	35s
posix (linux, gcc, static, ASan)	16s
posix (macos, clang, shared)	1m 6s
posix (macos, clang, static)	26s
posix (macos, clang, static, ASan)	1m 52s
	10m 5s

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