Remove FileAddress from layaout interfaces

[magicDGS]

To make the interface independent of our FileAddress object, we should return a number value instead of the object itself to allow other implementations to get rid of our abstraction. There are two possibilities:

• Return a long. This will allow to seek directly a file by using the returned value, and it will enforce a maximum value for the implementations. We are not planning to support pointers larger than a long yet (I have to think how that can be done).
• Return a Number. Like that, some implementations can use BigIntegeror their own number implementation: e.g., our FileAddress can extend Number to be used directly. In this case, we should point in our javadoc that the number will be converted to long for being seek, so arithmetic exceptions should be considered.

I prefer the first point, because like that all the enforcing is implementation-dependent. Something similar will be done with the object sizes.

[magicDGS]

I opted for adding an interface instead of a any of the previous options, to have more control over how the layout works in other implementations. See #28.