Implement local transitions

[Rich-Harris]

I was dreading this task but it turned out to be fairly simple. With this change, we call .i() and .o() methods on fragments with a 1 argument if the change is local (i.e. the call is in a p aka update method). Then, local transitions are wrapped in an if (local) ... block.

Local transitions present an opportunity to generate more optimal code — in a situation like this...

{#if x}
  {#if y}
    <div in:foo|local>...</div>
  {/if}
{/if}

...the outer block doesn't need any transition code at all. Am in two minds about whether to try and implement that now or worry about it later.

This PR is dedicated to @jacwright 😀

[codecov-io]

Codecov Report

Merging #2008 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #2008   +/-   ##
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  Coverage     100%    100%           
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  Files           1       1           
  Lines          48      48           
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  Hits           48      48

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