feat(go): generate new Size interface

The newly defined `Size` interface allows to calculate the encoded size
of an instance and pre-allocate a byte vector. It can be helful for
significantly large payloads that would do several re-allocations of a
buffer otherwise.

crates/stef-go/src/definition.rs

@@ -5,7 +5,7 @@ use stef_parser::{
     Schema, Struct, Type, TypeAlias, Variant,
 };
 
-use crate::{decode, encode, Opts, Output};
+use crate::{decode, encode, size, Opts, Output};
 
 /// Take a single schema and convert it into Go source code (which can result in multiple files).
 #[must_use]
@@ -68,9 +68,10 @@ fn render_definition<'a>(buf: &mut String, definition: &'a Definition<'_>) -> Op
             .unwrap();
             writeln!(
                 buf,
-                "\n{}\n{}",
+                "\n{}\n{}\n{}",
                 encode::RenderStruct(s),
-                decode::RenderStruct(s)
+                decode::RenderStruct(s),
+                size::RenderStruct(s),
             )
             .unwrap();
         }
@@ -709,7 +710,7 @@ impl Display for RenderEnumVariant<'_> {
 
         write!(
             f,
-            "\n{}\n{}",
+            "\n{}\n{}\n{}",
             encode::RenderEnumVariant {
                 enum_name: self.enum_name,
                 generics: self.generics,
@@ -720,6 +721,11 @@ impl Display for RenderEnumVariant<'_> {
                 generics: self.generics,
                 variant: self.variant,
             },
+            size::RenderEnumVariant {
+                enum_name: self.enum_name,
+                generics: self.generics,
+                variant: self.variant,
+            },
         )
     }
 }

crates/stef-go/src/encode.rs

@@ -110,7 +110,7 @@ impl Display for RenderFields<'_> {
                         writeln!(
                             f,
                             "\tw = buf.EncodeFieldOption[{}](w, {}, &v.F{idx}, func (w []byte, v \
-                             {0}) [] byte {{\n\t\treturn {}\n\t}})",
+                             {0}) []byte {{\n\t\treturn {}\n\t}})",
                             definition::RenderType(ty),
                             field.id.get(),
                             RenderType {

crates/stef-go/src/lib.rs

@@ -5,6 +5,7 @@ pub use definition::render_schema;
 mod decode;
 mod definition;
 mod encode;
+mod size;
 
 /// Options for the code generator that can modify the way the code is generated.
 #[derive(Default)]