Implement the remaining valid spec tests.

And lots of other miscellaneous changes. Rename InstanceWorld to
InstancePlus and reorganize its contents. This still isn't a great name,
but at least now it has a clear purpose.

misc_testsuite/stack_overflow.wast

@@ -0,0 +1,26 @@
+(module
+  (func $foo
+    (call $foo)
+  )
+  (func (export "stack_overflow")
+    (call $foo)
+  )
+)
+
+(assert_exhaustion (invoke "stack_overflow") "call stack exhausted")
+(assert_exhaustion (invoke "stack_overflow") "call stack exhausted")
+
+(module
+  (func $foo
+    (call $bar)
+  )
+  (func $bar
+    (call $foo)
+  )
+  (func (export "stack_overflow")
+    (call $foo)
+  )
+)
+
+(assert_exhaustion (invoke "stack_overflow") "call stack exhausted")
+(assert_exhaustion (invoke "stack_overflow") "call stack exhausted")