Waksman multiplication for gr_mat (#2109) #2870

Job	Run time
Alpine Linux, musl, 32-bit (assert, x1.5)	14m 4s
GCC build regression check (no tests)	1m 12s
macOS-M1, Clang (x3)	8m 15s
macOS-x86 Clang with BLAS (x2)	15m 24s
MinGW GCC (x0.5)	10m 46s
MSVC (x1)	9m 4s
Nemo.jl (temporary branch)	17m 51s
Ubuntu Clang with examples (x5)	13m 4s
Ubuntu GCC, Code Coverage (x10)	31m 56s
Ubuntu GCC with NTL (assert, x2)	8m 36s
	2h 10m 12s

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