counting and summing Bools with a small integer init promote to Int

[mbauman]

This is a small but concrete and independent change that can easily be split out from #58241 and is required for more pairwise reassociations (like #52397).

Specifically, it's required because count and sum use Base.add_sum, which very intentionally promotes small integers to Int or Uint. And if you add two Bools together, they also promote to Int. But if an init=0x00 is provided, then we get into a strange situation. This is the status quo on master right now:

julia> const +ₛ = Base.add_sum
add_sum (generic function with 4 methods)

julia> (((0x00 +ₛ true) +ₛ true) +ₛ true) +ₛ true
0x04

julia> ((0x00 +ₛ true) +ₛ true) +ₛ ((0x00 +ₛ true) +ₛ true)
0x0000000000000004

In other words, if we happen to choose to re-associate the branches here, we end up with a 0x02 +ₛ 0x02, which promotes to Uint.

This change here adds Bool into the extra promotion behaviors for add_sum. So with this commit, the above session becomes:

julia> (((0x00 +ₛ true) +ₛ true) +ₛ true) +ₛ true
0x0000000000000004

julia> ((0x00 +ₛ true) +ₛ true) +ₛ ((0x00 +ₛ true) +ₛ true)
0x0000000000000004

[mbauman]

OK, so upon reflection here, I think we should just always promote all small integers to Int. In fact, that's what this PR branch Julia (on both current master and this branch!) would do if the mapreduce machinery consistently called reduce_first.

[mbauman]

@nanosoldier runtests()

[nanosoldier]

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