✨ Añade macro para componer HTML en código

- Incorpora el código de la versión 0.25.0 de `maud_macros` creado por
  Chris Wong.
- Y reexporta los elementos esenciales desde la librería principal.

helpers/pagetop-macros/src/maud.rs

@@ -0,0 +1,39 @@
+// #![doc(html_root_url = "https://docs.rs/maud_macros/0.25.0")]
+// TokenStream values are reference counted, and the mental overhead of tracking
+// lifetimes outweighs the marginal gains from explicit borrowing
+// #![allow(clippy::needless_pass_by_value)]
+
+mod ast;
+mod escape;
+mod generate;
+mod parse;
+
+use proc_macro2::{Ident, Span, TokenStream, TokenTree};
+use proc_macro_crate::{crate_name, FoundCrate};
+use quote::quote;
+
+pub fn expand(input: TokenStream) -> TokenStream {
+    let output_ident = TokenTree::Ident(Ident::new("__maud_output", Span::mixed_site()));
+    // Heuristic: the size of the resulting markup tends to correlate with the
+    // code size of the template itself
+    let size_hint = input.to_string().len();
+    let markups = parse::parse(input);
+    let stmts = generate::generate(markups, output_ident.clone());
+
+    let found_crate = crate_name("pagetop").expect("pagetop is present in `Cargo.toml`");
+    let pre_escaped = match found_crate {
+        FoundCrate::Itself => quote!(
+            crate::html::PreEscaped(#output_ident)
+        ),
+        _ => quote!(
+            pagetop::html::PreEscaped(#output_ident)
+        ),
+    };
+
+    quote!({
+        extern crate alloc;
+        let mut #output_ident = alloc::string::String::with_capacity(#size_hint);
+        #stmts
+        #pre_escaped
+    })
+}