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.cc

@@ -782,7 +782,7 @@ class MFProcedureDotExport {
   void instruction_to_string(const MFCallInstruction &instruction, std::stringstream &ss)
   {
     const MultiFunction &fn = instruction.fn();
-    this->instruction_name_format(fn.name() + ": ", ss);
+    this->instruction_name_format(fn.debug_name() + ": ", ss);
     for (const int param_index : fn.param_indices()) {
       const MFParamType param_type = fn.param_type(param_index);
       const MFVariable *variable = instruction.params()[param_index];