Refine backprop assignments in error message (#87)

This PR refines the error message by incorporating information about the
assignments. Following is an example of an informative error message:

Code snippet:
```
func test(x *int) {
	x = nil
	y := x
	z := y
	print(*z)
}
```

Error message:
```
Potential nil panic detected. Observed nil flow from source to dereference point: 
   -> errormessage/errormessage.go:32:9: literal `nil` dereferenced via the assignment(s):
        -> `nil` to `x` at errormessage/errormessage.go:29:2,
        -> `x` to `y` at errormessage/errormessage.go:30:2,
        -> `y` to `z` at errormessage/errormessage.go:31:2
```

[closes #83 ]
[depends on #86 ]