Fix integer widening in sum and prod

[vera-rubin]

sum and prod on narrow integer static arrays can wrap instead of widening:

# Julia 1.10.10, StaticArrays 1.9.23
using StaticArrays
sum(SVector{2,Int8}(100,28))  # Int8(-128)
prod(SVector{2,Int8}(16,16))  # Int8(0)

Base documents Int results of 128 and 256 for these inputs, with unsigned integers widening to UInt.

We use Base.add_sum and Base.mul_prod in the static overloads so these reductions widen consistently with Base. We've added regression coverage for signed and unsigned integers, mapped calls, empty and singleton vectors, and dimension reductions, and ran the mapreduce and accumulate tests before and after the fix.

— ChipLab