simdarray: simplify code with a constexpr function

replace boolean trait type with a constexpr function

Signed-off-by: Matthias Kretz <kretz@kde.org>
simdarray: redesigned the Split helper

Split::lo/hi are not overloaded directly anymore. Now lo/hi will forward
to loImpl/hiImpl if those exist for the given argument type. Otherwise
they simply return what they got. This solves the problem of the
universal reference being overly greedy.

Signed-off-by: Matthias Kretz <kretz@kde.org>
dox: start/improve simdarray documentation

Creates a new Module/Group for simdarray and basically places the
complete simdarray headers into that group.

Signed-off-by: Matthias Kretz <kretz@kde.org>