Blender Git Loki

Nodes: use extern templates for socket declarations

The new socket declaration api generates a surprising amount
of symbols in each translation unit where it is used. This resulted
in a measurable compile time increase.

This commit reduces the number of symbols that are generated in
each translation unit significantly. For example, in
`node_geo_distribute_points_on_faces.cc` the number of symbols
decreased from 1930 to 1335. In my tests, this results in a 5-20%
compile time speedup when this and similar files are compiled
in isolation (measured by executing the command in `compile_commands.json`).

Compiling the distribute points on faces node sped up from ~2.65s to ~2.4s.