Ignore unexported struct field

There is an error when users try to compare structs that has unexported
field inside, although it's not being used. To fix the issue, I ignore
unexported fields when comparing the struct.

Find object ID for non-changed diff

Previously, both generate new and removed diff were not generating the
object ID field in their diff. Since it will be needed to get the object
ID there, I add the object ID finder in both of them.

Move comparator unit test folder

To get a better code coverage report, I move the comparator unit test to
the same package with the comparator itself. No logic change.

Fix generate change diff on Struct

Since we have the filteredValue variable, we need to use the same
variable when we are trying to generate the Diffs. So I fix it by
changing the variable that need to be compared.

Refactor Compare function (#15)

I start to feel the pain when trying to enhance the code to cover the edge cases. So, I decided to refactor the Compare function by separating each of the kind comparators. I also move the Diff struct from libra package to diff package, to avoid cyclic between the library.

Add capabilities to compare nested base type (#13)

There is a possibilities to compare a struct or map with the nested struct or map in it.

So I add the capabilities to compare nested struct and map