Refactor — eliminate 'hidden classes'

[Rich-Harris]

This Rollup issue got me thinking. #992 moved away from the concept of 'visitors' to the current approach, whereby AST nodes are converted into classes that implement common methods like build alongside more specialised ones.

It works, but:

• it means we have to clone the AST, so that it can be included in the svelte.compile output
• there are performance downsides to changing prototypes
• we're locked in to the structure of the AST generated by svelte.parse, which isn't necessarily the best structure for later phases (e.g. bindings and event handlers are lumped in with attributes). I don't think there's any guarantee that the parser could create the optimal structure

So what I propose, in essence, is that instead of augmenting the existing AST nodes we create entirely new objects. Instead of

Object.setPrototypeOf(node, EachBlock.prototype);

it would be

new EachBlock(node);

and the EachBlock constructor would be responsible for instantiating its children.