Duktape 2.3.0

[jessedoyle]

• Update duktape to version 2.3.0.
• Fix a missing it block in specs so that the expectation correctly executes.
• Add bindings and tests for new Duktape API functions:
  • random
  • push_new_target
  • get_global_heapptr
  • put_global_heapptr
• Update version to 0.15.0.
• Simplify README by removing wording for legacy Crystal versions.

Upstream release notes: https://github.com/svaarala/duktape/blob/5a6f30caabc8a856b113577fffd3468e8dfab621/doc/release-notes-v2-3.rst