Parse whitespace inside of entries

[stasm]

For serialization completeness, it could be helpful to store the whitespace inside of Entries. It should probably be stored as fields on SyntaxNode, e.g. space.before and space.after, with overlapping space stored twice.

See #116 for storing the top-level whitespace between entries.

[Pike]

Wondering if that's always a single syntax node that can have just two whities, but we'll figure that out once we get there.