Introduce generic 'Result' class

Useful to encapsulate the function result object (in case of having it) or, in case of failure, the failure reason.

This let us clean lot of boilerplate code, as now instead of returning a boolean and having to add a ref arg for the
return object and another ref for the error string. We can simply return a 'BResult<Obj>'.

Example of what we currently have:
```
bool doSomething(arg1, arg2, arg3, arg4, &result, &error_string) {
    do something...
    if (error) {
        error_string = "something bad happened";
        return false;
    }

    result = goodResult;
    return true;
}
```

Example of what we will get with this commit:
```
BResult<Obj> doSomething(arg1, arg2, arg3, arg4) {
    do something...
    if (error) return {"something happened"};

    // good
    return {goodResult};
}
```

This allows a similar boilerplate cleanup on the function callers side as well. They don't have to add the extra
pre-function-call error string and result object declarations to pass the references to the function.

src/Makefile.am

@@ -277,6 +277,7 @@ BITCOIN_CORE_H = \
   util/overloaded.h \
   util/rbf.h \
   util/readwritefile.h \
+  util/result.h \
   util/serfloat.h \
   util/settings.h \
   util/sock.h \