pre2.3.0 change 99: Add SLang_assoc_key_exists
pre2.3.0 change 100: Add SLstack_exch and tweak json to use it.
pre2.3.0 change 101: Made SLcompute_string_hash public and used it in the json module.
Merge branch 'manfred' into test to get json indent updates.
pre2.3.0 change 102: Version bump for manfred's updates (docs + json output indentation)