This reverts commit 3aba009c.

so that Holstein will compile

malloc_varsize_raw: cast pointer after malloc to correct type
ptr_nonzero: cast NULL to correct type before comparison
malloc_array_of_void: cast pointer after malloc

This reverts commit 9bfd9710.

default opaque type to void; use NULL of ref<void> to compare nullity of pointer to reduce the number of NULL constants

fix: the logic of block exit analysis has changed, mu_ccall is not explicitly identified and dealt with, thus it is possible now to cast pointer to correct type after mu_ccall(malloc). This should also fix issue#75 on Zebu issue tracker