Instead of depending on static initialization order of globals use
static variables within functions. Those are initialized on first use.
This is every so slighly less efficient, but avoids a full class of problems.
This adds the `MultiFunction` type and some smallish utility types that it uses.
A `MultiFunction` encapsulates a function that is optimized for throughput by
always processing many elements at once.
This is an important part of the new particle system, because it allows us to
execute user generated node trees for many particles efficiently.
Reviewers: brecht
Differential Revision: https://developer.blender.org/D8030
