Subtree memorization / reactive templates (#488)

This commit adds subtree memoization to Dioxus.

Subtree memoization is basically a compile-time step that drastically 
reduces the amount of work the diffing engine needs to do at runtime by
extracting non-changing nodes out into a static "template." Templates 
are then understood by the various renderers in the ecosystem as a 
faster way of rendering the same items. 

For example, in the web, templates are simply a set of DOM Nodes created 
once and then cloned later. This is the same pattern frameworks like Lithtml
and SolidJS use to achieve near-perfect performance. 

Subtree memoization adds an additional level of complexity to Dioxus. The RSX
macro needs to be much smarter to identify changing/nonchanging nodes and
generate a mapping between the Template and its runtime counterparts.

This commit represents a working starter point for this work, adding support 
for templates for the web, desktop, liveview, ssr, and native-core renderers.
In the future we will try to shrink code generation, generally improve 
performance, and simplify our implementation.

packages/autofmt/src/component.rs

@@ -167,7 +167,12 @@ impl Buffer {
                     write!(self.buf, "{}: {}", name, out)?;
                 }
                 ContentField::Formatted(s) => {
-                    write!(self.buf, "{}: \"{}\"", name, s.value())?;
+                    write!(
+                        self.buf,
+                        "{}: \"{}\"",
+                        name,
+                        s.source.as_ref().unwrap().value()
+                    )?;
                 }
                 ContentField::OnHandlerRaw(exp) => {
                     let out = prettyplease::unparse_expr(exp);
@@ -209,7 +214,7 @@ impl Buffer {
         let attr_len = fields
             .iter()
             .map(|field| match &field.content {
-                ContentField::Formatted(s) => s.value().len() ,
+                ContentField::Formatted(s) => s.source.as_ref().unwrap().value().len() ,
                 ContentField::OnHandlerRaw(exp) | ContentField::ManExpr(exp) => {
                     let formatted = prettyplease::unparse_expr(exp);
                     let len = if formatted.contains('\n') {