Various quality-of-life improvements (ideas from @33577 )

* Very basic `hello_main.c`
* Big starting hint to `write_c`
* Better error messages on failed test
  * TODO: also improve it for asmgen, but not sure how to do it in a good way

src/c/ast_equiv.rs

@@ -522,13 +522,3 @@ impl IsEquiv for AlignOf {
         self.0.is_equiv(&other.0)
     }
 }
-
-pub(crate) fn assert_ast_equiv(lhs: &TranslationUnit, rhs: &TranslationUnit) {
-    if !lhs.is_equiv(rhs) {
-        panic!(
-            r#"assertion failed: `(left.is_equiv(right))`
-             left: `{lhs:?}`,
-            right: `{rhs:?}`"#
-        )
-    }
-}

src/c/mod.rs

@@ -2,5 +2,4 @@ mod ast_equiv;
 mod parse;
 mod write_c;
 
-pub(crate) use ast_equiv::assert_ast_equiv;
 pub use parse::Parse;

src/c/write_c.rs

@@ -18,6 +18,7 @@ impl<T: WriteString> WriteString for Node<T> {
 }
 
 impl WriteLine for TranslationUnit {
+    /// VERY BIG HINT: You should start by understanding the [`writeln!`](https://doc.rust-lang.org/std/macro.writeln.html) macro.
     fn write_line(&self, indent: usize, write: &mut dyn Write) -> Result<()> {
         todo!()
     }