mulmod_shoup and nmod_vec_scalar_mul_nmod #2790

Job	Run time
macOS-x86 Clang with BLAS (x2)	25m 19s
macOS-M1, Clang (x3)	7m 29s
GCC build regression check (no tests)	1m 1s
Alpine Linux, musl, 32-bit (assert, x1.5)	14m 5s
MinGW GCC (x0.5)	10m 31s
MSVC (x1)	24m 19s
Nemo.jl (temporary branch)	11m 50s
Ubuntu Clang with examples (x5)	12m 49s
Ubuntu GCC, Code Coverage (x10)	30m 37s
Ubuntu GCC with NTL (assert, x2)	8m 32s
	2h 26m 32s

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