Port powmod from fq_poly to gr_poly, remove duplicates #271

Job	Run time
GCC build regression check (no tests)	1m 3s
Alpine Linux, musl, 32-bit (assert, x1.5)	13m 59s
MinGW GCC (x0.5)	10m 37s
macOS-M1, Clang (x3)	12m 54s
MSVC (x1)	8m 42s
Nemo.jl	18m 17s
Ubuntu Clang, BLAS and examples (x5)	12m 38s
Ubuntu GCC, Code Coverage (x10)	28m 43s
Ubuntu GCC with NTL (assert, x2)	8m 2s
	1h 54m 55s

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