Move self_less to the is_less callable #2561

Job	Run time
arm (clang, aarch64, -Wno-psabi)	31m 40s
externals (clang, x86, -msse2)	34m 12s
arm (clang, arm, -Wno-psabi)	33m 22s
externals (gcc, x86, -msse2)	51m 9s
macosx (clang, x86_osx, -mavx)	1h 27m 54s
no_pch (clang, x86, -msse2)	40m 30s
nosimd (clang, x86, -DEVE_NO_SIMD)	39m 13s
no_pch (gcc, x86, -msse2)	57m 37s
riscv (clang, riscv, -mrvv-vector-bits=128)	34m 45s
nosimd (gcc, x86, -DEVE_NO_SIMD)	44m 38s
x86-clang (clang, x86, -msse4.2)	10m 30s
x86-clang (clang, x86, -mavx)	12m 32s
x86-clang (clang, x86, -mavx2)	12m 36s
other-arch	0s
x86-gcc	0s
avx512 (clang, x86, -march=skylake-avx512)	29m 20s
avx512 (gcc, x86, -march=skylake-avx512)	47m 28s
other-cases	0s
	9h 27m 26s

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