HMR hooks

[Axelen123]

This PR adds hooks needed for HMR support and should fix #2377. $capture_state() still returns bindings and handlers but injecting the state still works. I haven't made any tests for the new functions but I have modified the expected result of some existing tests. Here is a Webpack Demo.

If in the demo you get an error like:

ERROR in ./src/App.svelte
Module build failed (from ./node_modules/svelte-loader/index.js):
Error: Cannot find module 'css-tree/lib/parser/index.js'

Then you should run:

git clone "https://github.com/Axelen123/svelte.git#hmr-hooks"
cd svelte
npm install
npm link
cd ..
npm link svelte

[Axelen123]

This PR is broken. I will create a new one