Compiling with gcc -O3 added some more warnings, especially about aliasing.

By setting Word_t to size_t we fix some of them, but jmap_@name_firstval
in JMAP_DEFINE_PTRIDX_TYPE we need to use a real size_t rather than lazily
casting our index to a size_t *.

Gcc also spotted taht we used idx[NUM] in test/run-ptridx-type.c; allocate
that and make the usage explicit.