glib/gbytes: save small byte buffers inline

When dealing with small allocations it can save considerable cycles to do a single allocation for both the GBytes and the data by tacking it onto the end of the GBytes.

Care is taken to preserve the glibc expectation of 2*sizeof(void*) alignment of allocations at the expense of some padding bytes.

The degenerate case here is when you want to steal the bytes afterwards but that amounts to the same overhead as the status-quo.

Where this can help considerably is in GVariant building such as g_variant_new_int32() which allocates for the GVariant, the GBytes, and the int32 within the GBytes.

In a simple benchmark of using GVariantBuilder to create (ii) variants this saved about 10% in wallclock time.