Encode less metadata for proc-macro crates

[Aaron1011]

Currently, we serialize the same crate metadata for proc-macro crates as
we do for normal crates. This is quite wasteful - almost none of this
metadata is ever used, and much of it can't even be deserialized (if it
contains a foreign CrateNum).

This PR changes metadata encoding to skip encoding the majority of crate
metadata for proc-macro crates. Most of the Lazy<[T]> fields are left
completetly empty, while the non-lazy fields are left as-is.

Additionally, proc-macros now have a def span that does not include
their body. This was done for normal functions in #75465, but was missed
for proc-macros.

As a result of this PR, we should only ever encode local CrateNums
when encoding proc-macro crates. I've added a specialized serialization
impl for CrateNum to assert this.

[rust-highfive]

r? @petrochenkov

(rust_highfive has picked a reviewer for you, use r? to override)

[Aaron1011]

@bors try @rust-timer queue

[rust-timer]

Awaiting bors try build completion

[bors]

⌛ Trying commit c0a1bccc2956267914c3f843acf0552cf100025b with merge 8364b62485652df7e6f55f5d5b925082ad5b9094...

[bors]

☀️ Try build successful - checks-actions, checks-azure
Build commit: 8364b62485652df7e6f55f5d5b925082ad5b9094 (8364b62485652df7e6f55f5d5b925082ad5b9094)

[rust-timer]

Queued 8364b62485652df7e6f55f5d5b925082ad5b9094 with parent bbc6774, future comparison URL.

[rust-timer]

Finished benchmarking try commit (8364b62485652df7e6f55f5d5b925082ad5b9094): comparison url.

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. Please note that if the perf results are neutral, you should likely undo the rollup=never given below by specifying rollup- to bors.

Importantly, though, if the results of this run are non-neutral do not roll this PR up -- it will mask other regressions or improvements in the roll up.

@bors rollup=never

[Aaron1011]

@bors try @rust-timer queue

[rust-timer]

Awaiting bors try build completion

[bors]

⌛ Trying commit 2341efee2714b050e7144d1a6e5c678f4c6bc11e with merge 9d18afcee1a0378a6656c876de82486d528525e7...

[bors]

☀️ Try build successful - checks-actions, checks-azure
Build commit: 9d18afcee1a0378a6656c876de82486d528525e7 (9d18afcee1a0378a6656c876de82486d528525e7)

[rust-timer]

Queued 9d18afcee1a0378a6656c876de82486d528525e7 with parent c6ab8e5, future comparison URL.

[rust-timer]

Finished benchmarking try commit (9d18afcee1a0378a6656c876de82486d528525e7): comparison url.

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. Please note that if the perf results are neutral, you should likely undo the rollup=never given below by specifying rollup- to bors.

Importantly, though, if the results of this run are non-neutral do not roll this PR up -- it will mask other regressions or improvements in the roll up.

@bors rollup=never

[Aaron1011]

@bors try @rust-timer queue

[rust-timer]

Awaiting bors try build completion

[bors]

⌛ Trying commit bc917942284b2721f542ca4348a8706999d2d01a with merge a16f40b1158cae86e56721c06794efa04f3918ce...

[bors]

☀️ Try build successful - checks-actions, checks-azure
Build commit: a16f40b1158cae86e56721c06794efa04f3918ce (a16f40b1158cae86e56721c06794efa04f3918ce)

[Aaron1011]

cc @rust-lang/infra A perf run didn't get queued for some reason

[Aaron1011]

@rust-timer build a16f40b1158cae86e56721c06794efa04f3918ce

[rust-timer]

Queued a16f40b1158cae86e56721c06794efa04f3918ce with parent 59fb88d, future comparison URL.

[rust-timer]

Finished benchmarking try commit (a16f40b1158cae86e56721c06794efa04f3918ce): comparison url.

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. Please note that if the perf results are neutral, you should likely undo the rollup=never given below by specifying rollup- to bors.

Importantly, though, if the results of this run are non-neutral do not roll this PR up -- it will mask other regressions or improvements in the roll up.

@bors rollup=never

[petrochenkov]

r=me with nits addressed.

[Aaron1011]

@bors r=petrochenkov

[bors]

📌 Commit b965356 has been approved by petrochenkov

[bors]

⌛ Testing commit b965356 with merge 623fb90...

[bors]

☀️ Test successful - checks-actions, checks-azure
Approved by: petrochenkov
Pushing 623fb90 to master...