Move self_less to the is_less callable #2546

Job	Run time
riscv (clang, riscv, -mrvv-vector-bits=128)	34m 19s
nosimd (clang, x86, -DEVE_NO_SIMD)	39m 27s
nosimd (gcc, x86, -DEVE_NO_SIMD)	46m 53s
externals (clang, x86, -msse2)	45m 51s
arm (clang, aarch64, -Wno-psabi)	31m 49s
macosx (clang, x86_osx, -mavx)	1h 29m 50s
arm (clang, arm, -Wno-psabi)	33m 30s
no_pch (clang, x86, -msse2)	56m 42s
externals (gcc, x86, -msse2)	51m 22s
no_pch (gcc, x86, -msse2)	58m 29s
x86-clang (clang, x86, -msse4.2)	13m 59s
x86-clang (clang, x86, -mavx)	17m 28s
x86-clang (clang, x86, -mavx2)	16m 46s
other-arch (gcc, sve256, -Wno-psabi)	54m 2s
other-arch (gcc, sve512, -Wno-psabi)	1h 53m 5s
other-arch (gcc, ppc64, -Wno-psabi)	35m 26s
avx512 (clang, x86, -march=skylake-avx512)	38m 6s
avx512 (gcc, x86, -march=skylake-avx512)	49m 57s
other-cases (clang_libcpp, x86, -msse2)	23m 14s
other-cases (clang, x86, -mavx2 -mbmi2 -DEVE_USE_BMI_ON_AVX2)	25m 59s
other-cases (gcc, x86, -mavx2 -mbmi2 -DEVE_USE_BMI_ON_AVX2)	45m 31s
x86-gcc (gcc, x86, -msse4.2)	18m 1s
x86-gcc (gcc, x86, -mavx)	22m 13s
x86-gcc (gcc, x86, -mavx2)	21m 27s
	16h 23m 26s

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