merge from root

Faster js2python string conversion

add missing hiwire_decref, fixes issue #301

Fix #305: Fix PDF export from matplotlib

Fixup symlinks

Reorg cpython tests

fix iodide-project/iodide#1423: Don't crash if there is no Python code

Check packages against Pyodide ABI version

Modified ``Makefile`` and ``packages/Makefile`` to get ``PYODIDE_PACKAGE_ABI`` from ``Makefile.envs``
Made ``checkABI`` as a public API function.
Modified ``file_packager.py`` to check for the compatible ABI number.
Defined ABI number with which pyodide is built in Makefile.envs
Accepted ABI number in ``file_packager.py`` as an argument.
Modified ``buildpkg.py`` and ``buildall.py`` to account for the new argument.
Added the corresponding ``test_different_ABI`` test.

Update docs about using Pyodide from Iodide.

Fix preloading on Chrome

Upgrade emscripten to 1.38.22