Cleanup also the code (avoid the goto).

To make it clearer that Py_BUILD_ASSERT_EXPR(cond) cannot be used as
assert(cond).

 * Downcasting from Py_UCS4 to Py_UNICODE is wrong is Py_UNICODE is 16-bit
   wchar_t
 * Remove old special case in getstring(), unicode is now handled separetely

Move other various macros to pymcacro.h

Thanks Rusty Russell for having written these amazing C macros!

formatbuf is now an array of Py_UCS4, not of Py_UNICODE

It now returns the number of written characters on success.