Simplified and HTML lowered statement dumps

Initial import, adaptation, and cleanup of statement simplification, HTML dump. Original code by @zingales. This needs a review.
Actually to remain consistent you should leave the close_div() tag where it is, and instead modify the css for AllocateBody to not indent, like so: 
```div.AllocateBody { padding-left: 0px;} \n```
This is the same strategy i used for the block statement. I think this is the correct strategy because each Halide Expr/Stmt node has an html element class which contains everything that comes from that element. Moving the close_div() would break this convention and i think its a convention we should keep, since it makes it easier when writing JS and CSS.  

On top of that i don't think we should modify AllocateBody to become a span, since spans represent elements that don't end in a newline, and the AllocateBody should end in a newline.