Towards inplace iterators for exponents of multivariate polynomials etc.

[joschmitt]

Add an inplace option to the iterators of exponents, coefficients, terms and monomials of a multivariate polynomial.
This is still a draft. Is this the "interface" we want?
Regarding the new functions coeff!, term! and exponent_vector!: Should they be exported (I did it for now because monomial! is exported as well)? Do I need to implement an "abstract fall-back" in src/MPoly.jl?

Nemocas/Nemo.jl#2161 implements the (few) necessary functions for QQMPolyRingElem. Using those, I get the following timings in oscar-system/Oscar.jl#5483 :

# Previously
julia> @btime inv_c[n-k+1:n];
  3.628 s (28973402 allocations: 1.73 GiB)

# With inplace iterators
julia> @btime inv_c[n-k+1:n];
  1.356 s (2811 allocations: 279.24 KiB)

So that moves in the right direction.

[joschmitt]

This is a bit ugly, but it gets the monomials iterator to constant allocations. Otherwise it would allocate a new 1 all the time. Is this too much of a corner case?