Generic Toom-3 multiplication for gr_poly #2810

Job	Run time
Ubuntu GCC with NTL (assert, x2)	8m 26s
Ubuntu GCC, Code Coverage (x10)	29m 51s
macOS-x86 Clang with BLAS (x2)	21m 53s
GCC build regression check (no tests)	1m 2s
macOS-M1, Clang (x3)	10m 33s
Alpine Linux, musl, 32-bit (assert, x1.5)	13m 58s
MinGW GCC (x0.5)	10m 27s
MSVC (x1)	9m 18s
Nemo.jl (temporary branch)	11m 56s
Ubuntu Clang with examples (x5)	12m 38s
	2h 10m 2s

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