Start of implementation for serde_json but I'm going to change my approach.

The permutations are pretty intense, so I think I'm going to do the same design I did for comparison where I have a JsonNumber (but I'll call this one MathNumber and rename JsonNumber to ComparisonNumber), convert the types to that, and then do the math.

src/renderer/mod.rs

@@ -21,6 +21,7 @@ pub use context_element::Walkable;
 pub use errors::CompileError;
 pub use errors::RenderError;
 pub use errors::WalkError;
+pub use parameters_context::math_ints;
 pub use renderer::compile_template;
 pub use renderer::DustRenderer;
 pub use select_context::SelectContext;