mulmod_shoup and nmod_vec_scalar_mul_nmod #2777

Job	Run time
GCC build regression check (no tests)	1m 5s
Ubuntu GCC with NTL (assert, x2)	8m 35s
macOS-x86 Clang with BLAS (x2)	19m 9s
macOS-M1, Clang (x3)	9m 34s
Ubuntu GCC, Code Coverage (x10)	30m 29s
Alpine Linux, musl, 32-bit (assert, x1.5)	13m 57s
MinGW GCC (x0.5)	10m 32s
MSVC (x1)	9m 12s
Ubuntu Clang with examples (x5)	12m 30s
Nemo.jl (temporary branch)	12m 20s
	2h 7m 23s

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