cgo: implement rudimentary C array decaying

This is just a first step. It's not complete, but it gets some real
world C code to parse.

This signature, from the ESP-IDF:

    esp_err_t esp_wifi_get_mac(wifi_interface_t ifx, uint8_t mac[6]);

Was previously converted to something like this (pseudocode):

    C.esp_err_t esp_wifi_get_mac(ifx C.wifi_interface_t, mac [6]uint8)

But this is not correct. C array parameters will decay. The array is
passed by reference instead of by value. Instead, this would be the
correct signature:

    C.esp_err_t esp_wifi_get_mac(ifx C.wifi_interface_t, mac *uint8)

So that it can be called like this (using CGo):

    var mac [6]byte
    errCode := C.esp_wifi_get_mac(C.ESP_IF_WIFI_AP, &mac[0])

This stores the result in the 6-element array mac.

testdata/cgo/main.h

@@ -144,3 +144,7 @@ extern int cflagsConstant;
 // test duplicate definitions
 int add(int a, int b);
 extern int global;
+
+// Test array decaying into a pointer.
+typedef int arraydecay_buf3[4][7][2];
+void arraydecay(int buf1[5], int buf2[3][8], arraydecay_buf3 buf3);