add ithread support, const a table

my summary/discussion of this patch is at https://github.com/rurban/Cpanel-JSON-XS/issues/16#issue-31532369

with the decode_hexdigits consting, .rdata went from 0x1b60 to 1c60, .data from 0x140 to 0x38, the numbers are from before the MY_CXT stuff, most of the remaining 0x38 is the 4 static otrs, plus junk that MS VC includes in all binaries unless you tell it otherwise (I remove the junk in my Win32::API module).