add extra methods for gr_poly scalar multiplication #2959

Job	Run time
Ubuntu GCC, Code Coverage (x10)	29m 47s
Ubuntu GCC with NTL (assert, x2)	7m 59s
GCC build regression check (no tests)	1m 5s
Alpine Linux, musl, 32-bit (assert, x1.5)	14m 9s
macOS-M1, Clang (x3)	8m 15s
MinGW GCC (x0.5)	10m 29s
MSVC (x1)	9m 33s
Nemo.jl	17m 42s
Ubuntu Clang, BLAS and examples (x5)	12m 57s
	1h 51m 56s

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