Remove the AST structures

[kpdecker]

These are difficult to maintain as an external API because of the array parameter mapping and they are basically just JSON generators that can be moved into the parser without much loss in fidelity or API functionality.

Filing issue to see if community has any input prior to making these changes.

/cc @mmun

[kpdecker]

As an example, most use of the AST methods would be converted to follow patterns like this:

 content
-  : CONTENT -> new yy.ContentStatement($1, yy.locInfo(@$))
-  ;
+  : CONTENT {
+    $$ = {
+      type: 'ContentStatement',
+      original: $1,
+      value: $1,
+      loc: yy.locInfo(@$)
+    };
+  };

[mmun]

This would be nice.