Do not generate unique __Field for each struct

[Mingun]

As suggested in #2250 (review), this PR replaces __Field generation with common structs inside serde crate.

There are three different forms of a __Field struct, each one has its own struct:

• Field + FieldSeed -- for structs without flatten fields and without #[serde(deny_unknown_fields)]
• FieldStrong + FieldStrongSeed -- for structs without flatten fields and with #[serde(deny_unknown_fields)]
• FieldFlatten + FieldFlattenSeed -- for structs with flatten fields

[dtolnay]

Thanks for the PR — this will be useful for benchmarking. Could you please look into compile time impact and runtime impact on https://github.com/serde-rs/json-benchmark, and compile time on https://github.com/gluon-lang/lsp-types?

I am not going to merge this though. I am not on board with exposing public Field/FieldStrong right away, and I am not on board with derive for simple structs needing to use __private in a way that users cannot copy into their handwritten impls. This is discussed in the review you linked, with a recommendation for a path forward.

[Mingun]

Here is some results: comparison.zip.

Measured commits v1.0.183 (05a5b7e) and df12e82, which removes all possible variants of __Field for struct field names and __Field for representing variants of externally tagged enums.

In both cases serde was injected using [patch] section in root Cargo.toml of both projects:

[patch.crates-io]
serde = { path = "../serde/serde" }

The following commands was used (taken from this comment):

• json-benchmark:

  cargo clean
  $env:CARGO_INCREMENTAL=0; cargo check --timings --no-default-features --features all-files,parse-struct,lib-serde
  cargo clean
  $env:CARGO_INCREMENTAL=0; cargo build --timings --release --no-default-features --features all-files,parse-struct,lib-serde

  PR is about 0.3 sec. faster on checking / building of json-benchmark lib itself. native jemalloc library was not found on my machine, so I cannot provide runtime impact right now
• lsp-types:

  cargo clean
  $env:CARGO_INCREMENTAL=0; cargo check --timings
  cargo clean
  $env:CARGO_INCREMENTAL=0; cargo build --timings

  PR is about 3 sec. faster on checking / building of lsp-types itself

@nnethercote perhaps you are willing to test impact of this PR on your cases?