Preserve whitespace inside nodes if necessary

[Rich-Harris]

Fixes #608. Whitespace handling is moved out of the parser and into the compiler, where we can be a bit smarter.

Essentially, Svelte keeps track of what the next sibling is while 'preprocessing' nodes, and uses that knowledge to determine whether whitespace should be removed from the end of a fragment:

<h1>Hello <strong>{{name}}! </strong><span>How are you?</span></h1>

The ! text node is aware that its parent element's next sibling is a <span> element rather than a text node with leading whitespace, so it knows to preserve the space after the exclamation mark. This would also be true if the <strong> element was wrapped in one or more elements.