Fix NPE for "value" instances and optional fields

[Mingun]

Move all offsets to a dedicates Span class and remove offset, that should be represent offset of substream in a parent stream. This generally does not work, because Kaitai Struct has ability to set any IO stream for an instance. The proper fix should be on the generator and runtime size -- kaitai-io/kaitai_struct_java_runtime#26.

Introduce a getSpan() method for a node, which could return null in case of

• if node represents an optional and missed sequential field
• if node represents an optional and missed "parse" instance
• if node represents a "value" instance

@pfroud, FYI

[pfroud]

Looks good to me!

This PR is a better alternative to #7