Add support for marshalling void*

void pointers are sometimes used for tags, such as in GtkTreeIter,
so it is useful to have basic marshalling implemented.
Note that they're treated like integers, and marshalled as doubles
in 64-bit arches, so there is no memory management involved.

https://bugzilla.gnome.org/show_bug.cgi?id=700643
25 jobs for wip/gcampax/700643-allow-creating-custom-tree-models in 2 minutes and 8 seconds (queued for 7 seconds)
latest
Status Name Job ID Coverage
  Source Check
passed cppcheck #138741

00:00:35

passed cpplint #138742

00:00:44

passed eslint #138743

00:00:37

failed fedora #138740

00:02:08

 
  Test
skipped no_graphics #138746
skipped no_profiler #138747
skipped no_readline #138748
skipped sanitizer_gcc #138744
skipped with_logging #138745
 
  Manual
skipped armv8 #138762
allowed to fail manual
skipped codequality #138749
allowed to fail manual
skipped code_statistics #138750
allowed to fail manual
skipped coverage #138764
allowed to fail manual
skipped fedora_gcc #138752
allowed to fail manual
skipped flatpak #138758
allowed to fail manual
skipped installed_tests #138753
allowed to fail manual
skipped lts #138756
allowed to fail manual
skipped ppc64le #138763
allowed to fail manual
skipped sanitizer_clang #138751
allowed to fail manual
skipped ubuntu_clang #138755
allowed to fail manual
skipped ubuntu_gcc #138754
allowed to fail manual
skipped valgrind #138757
allowed to fail manual
skipped zeal_11 #138761
allowed to fail manual
skipped zeal_2 #138759
allowed to fail manual
skipped zeal_4 #138760
allowed to fail manual
 
Name Stage Failure
failed
fedora Source Check
No job log