embedding: remove eutil::fptr_is_null, fix #3967, add Option<> to fptrs

[zmike]

@jdm, @larsbergstrom

legday

[hoppipolla-critic-bot]

Critic review: https://critic.hoppipolla.co.uk/r/3169

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[zmike]

Updated

[jdm]

I left a critic review but apparently you force-pushed over this commit in the past :(

[zmike]

I saw the critic review and addressed the match suggestions (though I can't compile it now because cargo is broken).

I'm not going to fix the indentation issues now because 1) the entire file has this formatting 2) the entire file will become generated code in the near future. It's a waste of time.

[jdm]

Fair enough.