ffi: make UnsignedRWStrategy support ulong only values

The UnsignedRWStrategy converted the passed value to a signed int. By this
only integers expressable as long can be used. This commit changes the
implementation to work this ulong as intermediate values.
ffi: implement more methods of AbstractMemory

Implement *_array_of_* methods + the specialized values for put_string
and write_bytes.
ffi: tag some known errors
ffi: Ignore vararg tests with float param

MRI ruby ffi treads floats (only) for varargs as double. But Topaz float
is in all cases an alias for float32. The ffi specs depend on the exception
for floats on vargs.

As I have no idee why is exception was made and now real clue where it would
by needed in topaz, exclude as tests with varargs + float for now.
ffi: adjust ffi/library.rb due to module_eval limitations in Topaz.

In Topaz Module.module_eval does current not support accessing local variables
from parent frame. This features is used by FFI::Library.attach_variable.
This commits adds a workaround into ffi/library.rb until this is fixed in
Topaz.
ffi: fails autoreleasing tests as fails due to Topaz limitations

Topaz current does not implement ObjectSpace.add_finializer. Flag test
as it can not work.
ffi: implement FFI::StructLayout#alignment