Slim down the main snowflake SVG

Actions done roughly looks like:

 1. Remove layers, deduplicate ungroup
 2. Remove unused defs
 3. Inline gradient transformations
 3. Make original path a definition
 4. Define two <use>s, one for each gradient
 5. Add 6th instantiation, use new defs
 6. Alpha-rename for clarity
 7. Remove explicit default attrs (0, 100%, etc)
 8. Remove inkscape-isms

The last step was done using [1] as advertised in [2].

One one hand, most of these abstractions will not be preserved by future
edits in inkscape. One the other hand, the SVG is now so simple that
future edits can probably easily be done by hand, or with inkscape's XML
editor as a sort of in-between method.

[1]: http://jsfiddle.net/alexanderby/31nmr8f7
[2]: https://graphicdesign.stackexchange.com/questions/49626/what-program-makes-clean-plain-svg-files-from-inkscape