New types uword_t, sword_t

New types uword_t, sword_t

First commit in a series of changes for LLP64 platform support.

We have been using `(un)signed long' assuming that it is an integer
of the same size as a pointer.  On Windows x64 this assumption does
not hold, and `(un)signed long long' is required for the same
purpose.

Code will be updated to use our own types uword_t, sword_t instead,
which are defined to be synonyms of uintptr_t and intptr_t, respectively.

Note that uword_t is also synonymous with our older type
pointer_sized_uint_t, and technically (albeit not conceptually) the
same as lispobj.

Thanks to Anton Kovalenko.