Fix a bunch of crashes discovered by fuzzing cppfront (#1383)

* Three more fuzz crashes.

* Add regression test for crash 10.

* Error instead of crash if users write silly things in aliases.

* Fix assertion on comments near end of file.

* Disallow declarations of parents after functions.

* Fix more fuzz crashes.

* Rename error test cases to `*-error.cpp2`

* Minor tidying to fit house style

`exit(1)` -> `exit(EXIT_FAILURE)`

Branch and loop bodies are always enclosed in `{` `}` even when they contain a single line

---------

Co-authored-by: Herb Sutter <herb.sutter@gmail.com>

Author: threeifbyair

include/cpp2util.h

@@ -561,7 +561,7 @@ class contract_group {
         std::cerr << ": " << msg;
     }
     std::cerr << "\n";
-    std::terminate();
+    std::exit(EXIT_FAILURE);
 }
 
 auto inline cpp2_default = contract_group(

regression-tests/mixed-bugfix-for-double-pound-else-error.cpp2

@@ -0,0 +1,2 @@
+#else
+#else

regression-tests/pure2-bugfix-for-assert-capture-error.cpp2

@@ -0,0 +1,3 @@
+crash_10: (foo: i32) = {
+  assert( 10LL as i32 == foo$);
+}

regression-tests/pure2-bugfix-for-bad-capture-error.cpp2

@@ -0,0 +1,3 @@
+crash_10: (foo: i32) = {
+  assert( 10LL as i32 == foo$);
+}

regression-tests/pure2-bugfix-for-bad-decltype-error.cpp2

@@ -0,0 +1,4 @@
+crash_89: () = {
+    f := new<int>(0);
+    _ = f is decltype.f); 
+}

regression-tests/pure2-bugfix-for-bad-parameter-error.cpp2

@@ -0,0 +1 @@
+print: <T: type> (inout out: std::ostream=args: T) requires true = {}

regression-tests/pure2-bugfix-for-functions-before-superclasses-error.cpp2

@@ -0,0 +1,8 @@
+crash_m0b: type = {
+}
+
+crash_m0c: type = {
+  name: i32;
+  get_name: (this) -> i32 = { return name; }
+  this: crash_m0b;
+}

regression-tests/pure2-bugfix-for-invalid-alias-error.cpp2

@@ -0,0 +1,3 @@
+outer: type = {
+    x: requires true == 42;
+}

regression-tests/pure2-bugfix-for-late-comments.cpp2

@@ -0,0 +1,11 @@
+
+main: () -> int = {
+  x := crash_m0();
+  _ = x;
+}
+
+crash_m0: type = {
+  operator-: (this, _) -> int = 0;/* Comment starts here
+And continues here
+*/
+}

regression-tests/pure2-bugfix-for-naked-unsigned-char-error.cpp2

@@ -0,0 +1,3 @@
+main: () = {
+    e: unsigned char
+}