(backport) Add TkLengthOne() function, just as TclLengthOne() in Tcl,… #415

Job	Run time
build (gcc)	1m 38s
build (gcc, CFLAGS=-DTK_NO_DEPRECATED=1)	1m 53s
build (gcc, --disable-xft)	1m 39s
build (gcc, --disable-xss)	1m 51s
build (gcc, --enable-symbols)	1m 50s
test (gcc)	3m 45s
build (clang)	1m 51s
test (gcc, --disable-xft)	4m 7s
build (clang, CFLAGS=-DTK_NO_DEPRECATED=1)	2m 8s
test (gcc, --enable-symbols)	3m 22s
build (clang, --disable-xft)	1m 53s
build (clang, --disable-xss)	1m 58s
build (clang, --enable-symbols)	1m 27s
	29m 22s

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