Add #[derive(FromLua)] macro to opt-in into FromLua<T> where T: 'static + Clone (userdata type).

Future macro implementations will allow making T from Lua tables/other values.
Relates to #291.

mlua_derive/src/from_lua.rs

@@ -0,0 +1,32 @@
+use proc_macro::TokenStream;
+use quote::quote;
+use syn::{parse_macro_input, DeriveInput};
+
+pub fn from_lua(input: TokenStream) -> TokenStream {
+    let DeriveInput {
+        ident, generics, ..
+    } = parse_macro_input!(input as DeriveInput);
+
+    let where_clause = match &generics.where_clause {
+        Some(where_clause) => quote! { #where_clause, Self: 'static + Clone },
+        None => quote! { where Self: 'static + Clone },
+    };
+    let ident_str = ident.to_string();
+
+    quote! {
+      impl #generics ::mlua::FromLua<'_> for #ident #generics #where_clause {
+        #[inline]
+        fn from_lua(value: ::mlua::Value<'_>, lua: &'_ ::mlua::Lua) -> ::mlua::Result<Self> {
+          match value {
+            ::mlua::Value::UserData(ud) => Ok(ud.borrow::<Self>()?.clone()),
+            _ => Err(::mlua::Error::FromLuaConversionError {
+                from: value.type_name(),
+                to: #ident_str,
+                message: None,
+            }),
+          }
+        }
+      }
+    }
+    .into()
+}