Unify scalar, batch, and batch_bool in some places

[kalenedrael]

• Add batch_traits that unifies traits for scalar, batch, and batch_bool.
• Treat batch_bool as its own mask type.
• Add casts for scalar types.
• Add select() for batch_bool.
• Simplify abs implementation.

[kalenedrael]

FWIW, the goal is to make it easier to write generic code that works on scalar and batch types. For instance, this enables something like:

template <typename VecF, VecI, VecB>
VecF DoStuff(VecF fv, VecI iv, VecB cond) {
  using F = typename batch_traits<VecF>::scalar_type;
  auto cond_i = iv > 42;
  return select(batch_cast<F>(cond_i) & cond, fv + batch_cast<F>(iv), 0.0));
}

which can be used as DoStuff(float, int, bool) or DoStuff(batch<float>, batch<int>, batch_bool<float>).

Unifying batch_bool with batch enables code like this to work for either type (at least it would, if select supported batch_bool, which I will add):

template <typename Vec>
auto LoadSelect(bool *b, Vec x, Vec y) {
  using VecB = typename batch_traits<Vec>::mask_type;
  return select(VecB::load_unaligned(b), x, y);
}

Tangentially related: I wonder if, in general, batch_bool should be parameterized on vector size instead of the underlying type. It's a bit strange to say that a boolean can only be used on a particular scalar type. Unfortunately, at least on x86, there is a distinction between integer and floating point vector types, and on many older processors there is a performance penalty for crossing domains. This can basically be ignored on any newer processor, but it's something to keep in mind.

[serge-sans-paille]

• Add batch_traits that unifies traits for scalar, batch, and batch_bool.

I like that!

* `batch_cast` can now be used on `batch_bool` (because why not?)

I 'm not quite sure about this one. We currently have a conversion operator from batch_bool to batch and within that setup I find it confusing to have a batch_cast on a a batch_bool returning a batch_bool.

[kalenedrael]

True. Do you think it's better to allow implicit conversion between batch_bool of the same size?

[serge-sans-paille]

True. Do you think it's better to allow implicit conversion between batch_bool of the same size?

I'm not a big fan of this either :-/
Just because explicit is better than implicit.

[kalenedrael]

I'll remove it from this PR; I think the other features are enough for what I want at the moment.

The problem I'm trying to solve is that batch_bool_cast is fundamentally nonsensical. There's no conceptual reason the result of an integer comparison shouldn't be directly usable for conditioning float expressions, for instance. Unfortunately I don't have a good solution yet.

[kalenedrael]

Ok, select for batch_bool has been added. This is ready now; please let me know if you have any concerns.

[serge-sans-paille]

This seems sub-optimal in the case where we don't have access to a fast select, see https://godbolt.org/z/3GqaxTsbd