Optimize `FqFieldElem` construction

[fingolfin]

In oscar-system/Oscar.jl#4581 I am trying to optimize conversion between GAP and OSCAR finite field elems (FFE), and one of the bottlenecks is the construction of FqFieldElem from a vector of integer coefficients (a Vector{Int}).

This ends up calling the following Nemo function:

function (a::FqField)(b::Vector{<:IntegerUnion})
  da = degree(a)
  db = length(b)
  da == db || error("Coercion impossible")
  return a(parent(defining_polynomial(a))(b))
end

So this first constructs a polynomial from the Vector{Int} and then converts that into an FqFieldElem. It would be nice if this intermediate step could be skipped.

However I am not sure how feasible that is without "cheating" (i.e. doing different things depending on which underlying field implementation is used).