Generic Toom-3 multiplication for gr_poly #2807

Job	Run time
Ubuntu GCC with NTL (assert, x2)	8m 29s
Ubuntu GCC, Code Coverage (x10)	30m 0s
GCC build regression check (no tests)	1m 5s
Alpine Linux, musl, 32-bit (assert, x1.5)	14m 15s
macOS-M1, Clang (x3)	7m 49s
macOS-x86 Clang with BLAS (x2)	14m 9s
MinGW GCC (x0.5)	10m 45s
Nemo.jl (temporary branch)	12m 13s
MSVC (x1)	9m 32s
Ubuntu Clang with examples (x5)	12m 37s
	2h 0m 54s

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