Nodes: add Collection socket type

The implementation of this socket works almost the same as object sockets.

I did not define a color for this socket yet (it's black for now).

Merge branch 'master' into geometry-nodes

Geometry Nodes: correct modifier name when creating from node editor

The name should be the same as when the modifier is created in the
modifier tab of the properties editor.

Geometry Nodes: automatically reconnect when swapping inputs

This makes it easier to swap the inputs to the Join Geometry node.
The behavior is the same as in the Math node.

Functions: add move operations to CPPType

Those are sometimes needed when dealing with c++ types in a generic way.

Functions: add generic pointer class

This class represents a pointer whose type is only known at runtime.

Nodes: add geometry socket type

We still have to pick a color for this socket.

Ref T81848.

Functions: add float2 cpp type

This also adds a hash function for `float2`, because `CPPType`
expects that currently.

BLI: add missing const

Geometry Nodes: initial scattering and geometry nodes

This is the initial merge from the geometry-nodes branch.
Nodes:
* Attribute Math
* Boolean
* Edge Split
* Float Compare
* Object Info
* Point Distribute
* Point Instance
* Random Attribute
* Random Float
* Subdivision Surface
* Transform
* Triangulate

It includes the initial evaluation of geometry node groups in the Geometry Nodes modifier.

Notes on the Generic attribute access API

The API adds an indirection for attribute access. That has the following benefits:
* Most code does not have to care about how an attribute is stored internally.
This is mainly necessary, because we have to deal with "legacy" attributes
such as vertex weights and attributes that are embedded into other structs
such as vertex positions.
* When reading from an attribute, we generally don't care what domain the
attribute is stored on. So we want to abstract away the interpolation that
that adapts attributes from one domain to another domain (this is not
actually implemented yet).

Other possible improvements for later iterations include:
* Actually implement interpolation between domains.
* Don't use inheritance for the different attribute types. A single class for read
access and one for write access might be enough, because we know all the ways
in which attributes are stored internally. We don't want more different internal
structures in the future. On the contrary, ideally we can consolidate the different
storage formats in the future to reduce the need for this indirection.
* Remove the need for heap allocations when creating attribute accessors.

It includes commits from:
* Dalai Felinto
* Hans Goudey
* Jacques Lucke
* Léo Depoix