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/ir0/hello_main.ir

@@ -0,0 +1,13 @@
+
+fun i32 @main () {
+init:
+  bid: b0
+  allocations:
+
+
+block b0:
+  ret 1:i32
+
+block b1:
+  ret 1:i32
+}