Only update each blocks when their dependencies have changed

[Rich-Harris]

Ref #381. So far, I'm just refactoring the each-block code, because it's grown a little bit wild and woolly. The work done for this PR (identifying which dependencies a fragment has) should also support #472

[Rich-Harris]

Alright, I think this is ready to go — am going to start work on #472 in parallel.

This PR involves a bit of a shift in how things happen. We first traverse the markup AST in a 'preprocess' step, which creates blocks (as before, with block.child(...)) but then attaches them to the node in question as node_block. Any time an expression is encountered (in a tag, a binding, an attribute, an if block or an each block), the expression's dependencies are added to the current block (and all parent blocks — so the top-level block has the largest number of dependencies).

This way, by the time we visit the node, we already know if it contains any dynamic content, and which properties to check for changes before updating if it does have dynamic content.