Add support for Tcl 9

Tcl 9 changed several API functions to take Tcl_Size, which is
ptrdiff_t, instead of int, for 64-bit enablement. We have to change a
few local variables to be compatible with that. We also provide a
fallback typedef of Tcl_Size for older Tcl versions. Here is a patch
for that.

The affected variables are used for quantities that will not approach
values beyond the range of int (usually number of columns), so this
doesn't change any functionality, AFAICT.