Cleanup: remove unnecessary calls to as_span

This uses the new implicit conversions and constructors
that have been committed in the previous commit.

I tested these changes on Linux with gcc and on Windows.

BLI: improve constructors and conversions to span

This allows us to avoid many calls to `as_span()` methods. I will
remove those in the next commit. Furthermore, constructors
of Vector and Array can convert from one type to another now.

I tested these changes on Linux with gcc and on Windows.

Cleanup: remove unused function

This is not necessary in C++17 anymore.

Cleanup: use c++17 helper variable templates

Functions: support hashing MFDataType and CPPType

Cleanup: use different internal socket name

Functions: add AttributesRef class

This is the same as MutableAttributesRef, but the data in it cannot be changed.

BLI: add comparison operators and hash functions for float3, etc.

BLI: add disjoint set data structure

This can be used to find separate islands in meshes efficiently (as is
done in cycles already). Furthermore, this helps to implement some
algorithms on node trees more efficiently.

Cleanup: add correct license header to tests