mulmod_shoup and nmod_vec_scalar_mul_nmod #2788

Job	Run time
Ubuntu GCC with NTL (assert, x2)	8m 30s
macOS-M1, Clang (x3)	8m 8s
GCC build regression check (no tests)	1m 7s
Alpine Linux, musl, 32-bit (assert, x1.5)	14m 2s
MinGW GCC (x0.5)	10m 31s
MSVC (x1)	9m 7s
Ubuntu Clang with examples (x5)	12m 41s
Ubuntu GCC, Code Coverage (x10)	17m 10s
macOS-x86 Clang with BLAS (x2)	17m 18s
Nemo.jl (temporary branch)	11m 45s
	1h 50m 19s

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