mulmod_shoup and nmod_vec_scalar_mul_nmod #2783

Job	Run time
macOS-x86 Clang with BLAS (x2)	19m 7s
Alpine Linux, musl, 32-bit (assert, x1.5)	14m 5s
macOS-M1, Clang (x3)	8m 0s
GCC build regression check (no tests)	1m 3s
MinGW GCC (x0.5)	10m 31s
MSVC (x1)	9m 17s
Nemo.jl (temporary branch)	12m 53s
Ubuntu Clang with examples (x5)	12m 41s
Ubuntu GCC, Code Coverage (x10)	29m 54s
Ubuntu GCC with NTL (assert, x2)	8m 40s
	2h 6m 11s

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