Auto merge of #59953 - eddyb:soa-metadata, r=michaelwoerister

 rustc_metadata: replace Entry table with one table for each of its fields (AoS -> SoA).

In https://github.com/rust-lang/rust/pull/59789#issuecomment-481958212 I noticed that for many cross-crate queries (e.g. `predicates_of(def_id)`), we were deserializing the `rustc_metadata::schema::Entry` for `def_id` *only* to read one field (i.e. `predicates`).

But there are several such queries, and `Entry` is not particularly small (in terms of number of fields, the encoding itself is quite compact), so there is a large (and unnecessary) constant factor.

This PR replaces the (random-access) array¹ of `Entry` structures ("AoS"), with many separate arrays¹, one for each field that used to be in `Entry` ("SoA"), resulting in the ability to read individual fields separately, with negligible time overhead (in thoery), and some size overhead (as these arrays are not sparse).

In a way, the new approach is closer to incremental on-disk caches, which store each query's cached results separately, but it would take significantly more work to unify the two.

For stage1 `libcore`'s metadata blob, the size overhead is `8.44%`, and I have another commit (~~not initially included because I want to do perf runs with both~~ **EDIT**: added it now) that brings it down to `5.88%`.

¹(in the source, these arrays are called "tables", but perhaps they could use a better name)

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