Improve the sync design

[hrsh7th]

Currently, vsnip does only the placeholder syncing.

I think the variable resolving should realized by the same approach.

1. Does not convert the known variable-node to text-node.
2. Rename s:Variable.children to s:Variable.arguments (do not traverse it).
3. s:Snippet.init should gather the origin placeholders as origin_map.
4. s:Snippet.init should remove the derivation placeholders's children.
5. s:Variable.resolve should take the origin_map for resolving.
6. s:Placeholder should have resolve function.