[TEMPLATES] Change StructResolver instance approach

[TheRedDaemon]

Result of experimentation, since the MACRO approach lacked a test regarding structs.
Short result:

• If a relatively clean instance definition is wanted, the MACRO approach would require a condition check in both the header and the implementations file. I found no way around that.
• If templates are used, there are ways to place the instantiation in the header by partial definition of a instance template. This would require boilerplate.
• This MR uses the latter method to remove the instance-Method approach from the function resolver. While it needs to use another macro, the object construction now seems to happen without any additional side effect.