script: Use a 16-element `SmallVec` for the root collection.

In my tests the size of the root collection never exceeded 7, so 16
seems like a nice conservative number.
script: Avoid a temporary when constructing `<td>` and `<tr>` elements.

Was a huge improvement to RoboHornet.
script: Use `String::new()` instead of the formatting infrastructure
when constructing DOM unique IDs.

This was showing up in the RoboHornet profile.
script: Don't dirty nodes that are already dirty.
script: Don't create a temporary vector on the heap when inserting
non-fragment nodes.
script: Use an FNV hash to hash event listeners.

The security properties of SipHash are irrelevant for event listeners
and the creation of the random number generator was showing up high in
the profiles.
script: Fix busted document fragment appending code
auto merge of #3835 : pcwalton/servo/script-microoptzns, r=Ms2ger

This is a grab bag of various microoptimizations for script that I came across when profiling our performance on RoboHornet.

r? @jdm