Use fmpq_mul_si

[joschmitt]

I noticed that *(::QQFieldElem, ::Int) calls generic AbstractAlgebra code instead of flint.

Before

julia> a = QQ(1, 2)
1//2

julia> @btime 2*a;
  116.807 ns (11 allocations: 192 bytes)

After

julia> @btime 2*a;
  20.632 ns (1 allocation: 32 bytes)

This is related to thofma/Hecke.jl#1537 .