Improve teardown code by unifying detach branches

[mrkishi]

This PR groups detach statements into a single if branch.

Note that I haven't modified the teardown template to deal with empty lists: you can end up with extra newlines and an empty if in those cases. Should that be left for minifiers or should we handle them?

Closes #99.

[codecov-io]

Current coverage is 90.86% (diff: 100%)

Merging #110 into master will increase coverage by <.01%

@@             master       #110   diff @@
==========================================
  Files            48         48          
  Lines          1324       1325     +1   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits           1203       1204     +1   
  Misses          121        121          
  Partials          0          0          

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[Rich-Harris]

Much better, thank you!

Should that be left for minifiers or should we handle them?

I think Svelte should handle it, if only for the sake of readability. There's a few places throughout Svelte where the generated code could be tidied up a bit – probably not too high a priority though