This will be useful to add static assertion to some tests like mozilla#463 wants to do.

For example, in the case of the test for mozilla#463, the test would need something
like:

    trailer = """
    #include <assert.h>

    #if defined(CBINDGEN_STYLE_TAG) && !defined(__cplusplus)
    static_assert(sizeof(struct P) == 4, "unexpected size for P");
    #else
    static_assert(sizeof(P) == 4, "unexpected size for P");
    #endif
    """

As more of these tests are added it may be worth just adding a helper header
like this to avoid some duplication:

    #include <assert.h>
    #if defined(CBINDGEN_STYLE_TAG) && !defined(__cplusplus)
    #define CBINDGEN_STRUCT(name) struct name
    #else
    #define CBINDGEN_STRUCT(name) name
    #endif

And so on, so the previous configuration would become just:

    trailer = """
    #include "testing-helpers.h" // Or whatever
    static_assert(sizeof(CBINDGEN_STRUCT(P)) == 4, "unexpected size for P");
    """

That may or may not be overkill to do as part of mozilla#463.

This allows to assert easily as described in the previous commit.