Remove the macros for the g_malloc_n family -- calls directly to those
functions now always go directly to those functions.

Reimplement the macros for g_new and friends.

Remove the branch that checked for calling g_new() with a constant
n_structs == 1.  With the struct size always known this case will now be
caught under the case that does the inline multiplication and the
multiplication by 1 will be optimised away.