Add modular splitting #1270

Job	Run time
macOS GCC with BLAS (x3)	23m 1s
Ubuntu GCC, default flags with AVX2, Code Coverage (x10)	43m 52s
Cygwin GCC (x0.5)	46m 55s
MinGW GCC (x0.5)	39m 32s
FreeBSD Clang (x0.5)	18m 28s
MSVC (x1)	25m 53s
Nemo.jl	16m 16s
Ubuntu Clang (x5)	39m 22s
Ubuntu GCC via CMake (no check)	12m 11s
Ubuntu GCC (assert, x2)	24m 24s
	4h 49m 54s

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