Remove `BuiltinFunId`

[Nadrieril]

Today a function call may refer to a BuiltinFunId instead of a FunDecl item. This is annoying because clients need to special-case these, including knowing their signatures. They're all optional today, and to my knowledge only really used by Aeneas. I'd like to get rid of them and instead use the corresponding std functions:

• BoxNew is just Box::new;
• ArrayToSliceShared/ArrayToSliceMut are core::array::to_slice/to_slice_mut;
• ArrayRepeat is std::array::repeat;
• the various Index variants have equivalents involving the Index trait;
• PtrFromParts is std::ptr::from_raw_parts;
• BoxWrite is Box<MaybeUninit<T>>::write;
• SliceIntoVec is slice::into_vec.

Because these are for the most part introduced in post-translation passes, we'll need to figure out a way to trigger the translation of these functions during translation when needed. I'm thinking a map from lang_item to ItemId could come useful.