Fix issue unable to parse empty arrays

• Fix issue #19 - Unable to parse empty arrays with quotes
• Add regression tests for this issue

Remove namespaces

• Fix typo in EXPLAIN document
• Stop testing on broken Travis-CI environment
• Breaking changes
  • Remove all support for PicoLisp namespaces
  • Prefix all internal function names with json-
• Adjust tests to use new function names
• Remove tests for namespace issue #9
• Update "picolisp-unit" testing dependency to v3.0.0
• Update supported PicoLisp version to 18.12

Optionally prevent duplicate keys

• Optionally prevent duplicate keys from being accepted with '*Json_prevent_duplicate_keys'
• Remove optional 'Make' parameter for '(link-array)'

Store JSON parsing error message in *Msg

• Store JSON parsing error message in *Msg

Invalid parsing of certain characters with \uNNNN

• Fix issue #15 - Invalid parsing of certain characters with \uNNNN
• Add regression tests for this issue

Invalid encoding of control characters 0x01-0x1F

• Fix issue #13 - Invalid encoding of control characters 0x01-0x1F
• Add regression tests for this issue

Invalid encoding of quote and solidus

• Fix issue #12 - Invalid encoding of quote and solidus (\ and ") characters
• Add regression tests for this issue
• Fix existing regression tests which provided incorrect results

Invalid encoding of special control characters

• Fix issue #11 - Invalid encoding of special control characters (^J, ^M, ^I)
• Ensure ^H (\b) and ^L (\f) are also encoded correctly
• Add regression tests for this issue

Invalid parsing of strings with caret (^)

• Fix issue #10 - Invalid parsing of strings with caret (^)
• Add regression tests for this issue

Fix decoding with namespaces

• Fix issue #9 - Bug in namespaces functionality
• Add regression tests for this issue
• Add notice about namespaces in PicoLisp >= 17.3.4
• Don't load module.l in json.l