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

examples/c/temp2.c

@@ -1,29 +1,33 @@
-struct color { int number; char name; };
+struct color {
+    int number;
+    char name;
+};
 
 int main() {
-  int temp = 0;
-  temp += sizeof(unsigned char);
-  temp += _Alignof(unsigned char);
+    int temp = 0;
+    temp += sizeof(unsigned char);
+    temp += _Alignof(unsigned char);
 
-  struct color c = {1, 2};
-  temp += c.name;
-  struct color *cp = &c;
-  temp += cp->name;
+    struct color c = {1, 2};
+    temp += c.name;
+    struct color* cp = &c;
+    temp += cp->name;
 
-  for(int i = 0, j = 0; i < 10; ++i) {
-    if ( i == 2 && j == 0) break;
-    temp += i;
-  }
-
-  switch(temp) {
-    case 1: {
-      temp = 0;
-      break;
+    for (int i = 0, j = 0; i < 10; ++i) {
+        if (i == 2 && j == 0)
+            break;
+        temp += i;
     }
-    default: {
-      break;
-    }
-  }
 
-  return temp;
+    switch (temp) {
+        case 1: {
+            temp = 0;
+            break;
+        }
+        default: {
+            break;
+        }
+    }
+
+    return temp;
 }