more type-stable reductions

[stevengj]

This patch tries to make reductions, especially sum and prod, as type-stable as is feasible (#6069). Key strategies:

• For reductions of two or more elements, always initialize the reduction variable as v = op(A[1], A[2]) rather than as v = A[1].
• For summation of empty collections, return zero(T)+zero(T) if T is known, rather than zero(T). (Similarly for prod and one.)
• For summations of single-element collections, return A[1] + zero(A[1]) rather than A[1]. (Similarly for prod and one.)

sum and prod of empty collections for which the eltype is unknown now throw an exception rather than returning 0 (which is the wrong answer for non-Number collections).

cc: @jiahao, @lindahua, @StefanKarpinski

[stevengj]

(Note that this only affects reduce and friends. I haven't tried to patch up reducedim.)

[StefanKarpinski]

Looks good to me. Thanks for doing this.