feat(go_port): reproduce race condition with object and class

[Schrodinger257]

Description

In object (class) when calling get or set or call, GO passes o.ptr (c.ptr) to the c function. if the c function was doing a work that takes long time to execute before storing the pointer passed to it from GO and GO garbage collector ran while c function still working it will see the object (class) as unreachable and as a result it will call their finalizers and free them which causes a segfault when the c function tries to access it.

In this PR i reproduced the race condition as follows:
first creating a py script in memory and load py function that return a class. after it i ran a goroutine that calls garbage collector in the background many times and created an object from the class and called Get("width") on it which triggered the problem and caused a segfault

signal: segmentation fault (core dumped)
FAIL    github.com/metacall/core/source/ports/go_port/source    1.771s

using defer runtime.KeepAlive() tells the garbage collector that the object (class) must be kept alive during the execution of Get() function and never try to free or call its finalizer. and this solved the problem

width = 10
PASS
ok      github.com/metacall/core/source/ports/go_port/source    1.208s

Fixes in PR #875