Avoid polluting host Function prototypes

[ExE-Boss]

Fixes #52

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createCurrentUnsafeRec() is only called once from realm.js to create a unsafeRec for the host’s primal Realm:

realms-shim/src/realm.js

Line 169 in 067bdd9

const currentUnsafeRec = createCurrentUnsafeRec();

[ExE-Boss]

Looks like this will need further fixing to handle the makeCompartment() case.