Waksman multiplication for gr_mat #2869

Job	Run time
GCC build regression check (no tests)	1m 14s
macOS-M1, Clang (x3)	9m 59s
Alpine Linux, musl, 32-bit (assert, x1.5)	14m 18s
macOS-x86 Clang with BLAS (x2)	16m 27s
MinGW GCC (x0.5)	11m 17s
MSVC (x1)	9m 19s
Nemo.jl (temporary branch)	17m 28s
Ubuntu Clang with examples (x5)	12m 40s
Ubuntu GCC, Code Coverage (x10)	30m 9s
Ubuntu GCC with NTL (assert, x2)	8m 46s
	2h 11m 37s

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