add new kind of test: generating .h files. and more

* docgen supports obj_err code kind for demonstrating
   errors without explicit test cases
 * add documentation for `extern enum`. See #367
 * remove coldcc keyword and add @setIsCold. See #661
 * add compile errors for non-extern struct, enum, unions
   in function signatures
 * add .h file generation for extern struct, enum, unions

test/gen_h.zig

@@ -0,0 +1,53 @@
+const tests = @import("tests.zig");
+
+pub fn addCases(cases: &tests.GenHContext) {
+    cases.add("declare enum",
+        \\const Foo = extern enum { A, B, C };
+        \\export fn entry(foo: Foo) { }
+    ,
+        \\enum Foo {
+        \\    A = 0,
+        \\    B = 1,
+        \\    C = 2
+        \\};
+        \\
+        \\TEST_EXPORT void entry(enum Foo foo);
+        \\
+    );
+
+    cases.add("declare struct",
+        \\const Foo = extern struct {
+        \\    A: i32,
+        \\    B: f32,
+        \\    C: bool,
+        \\};
+        \\export fn entry(foo: Foo) { }
+    ,
+        \\struct Foo {
+        \\    int32_t A;
+        \\    float B;
+        \\    bool C;
+        \\};
+        \\
+        \\TEST_EXPORT void entry(struct Foo foo);
+        \\
+    );
+
+    cases.add("declare union",
+        \\const Foo = extern union {
+        \\    A: i32,
+        \\    B: f32,
+        \\    C: bool,
+        \\};
+        \\export fn entry(foo: Foo) { }
+    ,
+        \\union Foo {
+        \\    int32_t A;
+        \\    float B;
+        \\    bool C;
+        \\};
+        \\
+        \\TEST_EXPORT void entry(union Foo foo);
+        \\
+    );
+}