proc_macro/bridge: send diagnostics over the bridge as a struct

[mystor]

This removes some RPC when creating and emitting diagnostics, and
simplifies the bridge slightly.

After this change, there are no remaining methods which take advantage
of the support for &mut references to objects in the store as
arguments, meaning that support for them could technically be removed if
we wanted. The only remaining uses of immutable references into the
store are TokenStream and SourceFile.

r? @eddyb

[eddyb]

@bors r+ rollup=never

[bors]

📌 Commit 1c7c792 has been approved by eddyb

It is now in the queue for this repository.

[bors]

⌛ Testing commit 1c7c792 with merge 3892b70...

[bors]

☀️ Test successful - checks-actions
Approved by: eddyb
Pushing 3892b70 to master...

[rust-timer]

Finished benchmarking commit (3892b70): comparison URL.

Overall result: ❌✅ regressions and improvements - no action needed

@rustbot label: -perf-regression

Instruction count

This is a highly reliable metric that was used to determine the overall result at the top of this comment.

	mean1	range	count2
Regressions ❌ (primary)	0.3%	[0.3%, 0.3%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.6%	[-0.7%, -0.5%]	3
All ❌✅ (primary)	0.3%	[0.3%, 0.3%]	1

Max RSS (memory usage)

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean1	range	count2
Regressions ❌ (primary)	3.5%	[3.5%, 3.5%]	1
Regressions ❌ (secondary)	4.0%	[2.4%, 5.2%]	4
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-5.0%	[-6.3%, -3.7%]	2
All ❌✅ (primary)	3.5%	[3.5%, 3.5%]	1

Cycles

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean1	range	count2
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.1%	[-2.1%, -2.1%]	1
All ❌✅ (primary)	-	-	0

Footnotes

1. the arithmetic mean of the percent change ↩ ↩² ↩³

2. number of relevant changes ↩ ↩² ↩³