[wip] update to spidermonkey 17

update erlang_js to use spidermonkey version 17 (latest published) and nspr 4.9.3 (required by this version of spidermonkey)

the current status is:

it builds, the examples in the readme run, test pass .... when it doesn't segfault.

also there are some notes on the changes that may need to be discussed.

it would be cool if you helped to try the pull request and help me troubleshoot the segfault, it seems to be when the vm is shutting down.