Blender Git Loki

New data type for constant (!) strings.

The const-ness is very important: strings are only stored as const char*
on the stack, which point to memory inside the instructions list.
The instructions contain the actual char array, with a null terminator.

This does not allow allocating new strings during eval, but it makes
it possible to use static strings as identifiers and keys for global data.
Eventually we could allow string operations through an allocation system
similar to DerivedMesh handling, with init/release nodes to manage
alloc/free.