Converts tokens into an Abstract Syntax Tree.

AST node types:

• {:text, binary}
• {:var, keys, escaped?, formatter}
• {:section, keys, children}
• {:inverted, keys, children}
• {:partial, name, indent}
• {:line, children, newline?} — a physical line whose non-whitespace content is entirely enclosed in sections that open and close on that line; the renderer collapses it (newline included) when it renders blank