Functions: use static names for multi-functions

Previously, the function names were stored in `std::string` and were often
created dynamically (especially when the function just output a constant).
This resulted in a lot of overhead.

Now the function name is just a `const char *` that should be statically
allocated. This is good enough for the majority of cases. If a multi-function
needs a more dynamic name, it can override the `MultiFunction::debug_name`
method.

In my test file with >400,000 simple math nodes, the execution time improves from
3s to 1s.

source/blender/functions/intern/multi_function_procedure_executor.cc

@@ -20,10 +20,9 @@
 
 namespace blender::fn {
 
-MFProcedureExecutor::MFProcedureExecutor(std::string name, const MFProcedure &procedure)
-    : procedure_(procedure)
+MFProcedureExecutor::MFProcedureExecutor(const MFProcedure &procedure) : procedure_(procedure)
 {
-  MFSignatureBuilder signature(std::move(name));
+  MFSignatureBuilder signature("Procedure Executor");
 
   for (const ConstMFParameter &param : procedure.params()) {
     signature.add(param.variable->name(), MFParamType(param.type, param.variable->data_type()));