Minor style cleanups in Cache code

- Make ApplicableDeclarationsCache::cache priv
- Add a missing semicolon for clarity
- Remove a useless `drop` call.
Minor code cleanup in css::matching
Fix deriving in cascade_with_cached_declarations

Even though we don't recompute non-inherited properties here, we still need
to compute properties that are derived from them.  Fixes #2498: because
text-decoration is not inherited, -servo-text-decorations-in-effect was not
derived correctly for cached declarations.
auto merge of #2581 : mbrubeck/servo/declarations-cache, r=pcwalton

Fixes #2498.  Also includes a reftest, and some related code cleanup (in separate commits).

Note: The fix in `properties.rs.mako` is much more readable with `git diff -w`.