Update coreexttest.fth history

Updated to ersion 0.13.6

Replaced 2VARIABLE FP with CREATE 0 , 0 ,

Updated for new release 0.13.5

Updated to version 0.13.4

Update changes.txt

Lists changes made up to release 0.13.3

Version 0.13.2 changes

Changes made for release 0.13.1

blocktest.fth
   - fixed issue #7 faults with single buffer blocks
core.fth
   - corrected ACCEPT buffer block length to decimal 80 characters
coreplustest.fth 
   - moved double number with prefixes to doubletest.fth
   - now tests maximum length definition names and all graphical characters accepted
   - parses upper case characters using 'X' notation
   - tests FIND with zero length strings and non-existent names
doubletest.fth
   - received double number with prefixes from coreplus.fth

STR= replaced with S=

Should have been done before