mulmod_shoup and nmod_vec_scalar_mul_nmod #2796

Job	Run time
GCC build regression check (no tests)	1m 3s
Ubuntu GCC, Code Coverage (x10)	29m 34s
macOS-M1, Clang (x3)	7m 54s
Ubuntu GCC with NTL (assert, x2)	8m 27s
Alpine Linux, musl, 32-bit (assert, x1.5)	14m 5s
macOS-x86 Clang with BLAS (x2)	13m 36s
MSVC (x1)	9m 44s
MinGW GCC (x0.5)	10m 22s
Nemo.jl (temporary branch)	11m 37s
Ubuntu Clang with examples (x5)	12m 36s
	1h 58m 58s

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