Remove proc-macro-error as an abandonware dependency

[johngigantic]

I ran into an error in my CI/CD testing. I am performing a test where I compile all crate dependencies using the minimum valid version numbers. The bilge-impl crate fails to compile - although the error is in its dependency, the proc-macro-error crate. The proc-macro-error crate has a bug in version 1.0.0 which is fixed in its latest version, 1.0.4. However, because bilge-impl specifies the minimum proc-macro-error version as 1.0 instead of ^1.0.4, this error shows up when I do the minimum-version test.

The quick and easy fix is to specify the version number, and just change the bilge-impl Cargo.toml file dependency to proc-macro-error = { version = "1.0.4", default-features = false }

I think that the quick fix would be wrong, however - it's clear that the proc-macro-error crate is abandoned. It doesn't have any updates since 3 years ago, its owner has no git history over the last year, and it is failing to keep up with some critical libraries - syn 2.0, for example.

To finish this out, I think that
(1) any proc-macro-error usage should be taken out of the codebase.
(2) this library should also implement a similar minimum version CI/CD test.

I'd be happy to help with both and submit a PR.

[johngigantic]

My compile error, shown in my CI/CD pipeline

cargo check --all-features --locked
...
   Compiling bilge-impl v0.2.0
error: expected one of `.`, `;`, `?`, `else`, or an operator, found `::`
  --> /home/runner/.cargo/registry/src/index.crates.io-6f17d22bba15001f/bilge-impl-0.2.0/src/try_from_bits.rs:35:9
   |
35 | ...   emit_call_site_warning!("enum fills its bitsize"; help = "you can use `#[derive(FromBits)]` instead, rust will provide `TryFrom` for you (so you don't necessarily have to update call-sites)...
   |       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected one of `.`, `;`, `?`, `else`, or an operator
   |
   = note: this error originates in the macro `emit_call_site_warning` (in Nightly builds, run with -Z macro-backtrace for more info)

error: could not compile `bilge-impl` (lib) due to previous error
Error: Process completed with exit code 101.

[johngigantic]

Steps to replicate:

• Get in the bilge-impl directory
• Set the compiler to use the nightly toolchain - the next step depends on this.
• Run cargo generate-lockfile -Z minimal-versions to generate a lockfile with the minimum possible version numbers
• (Alternatively, edit the cargo.toml file to specifically pin proc-macro-error V1.0.0)
• Run cargo build

[hecatia-elegua]

will see what they're doing and maybe fork or find a fork, thank you for the heads-up!
There are some forks already.

(for later use, this is the original:)
https://crates.io/crates/proc-macro-error/1.0.4
https://gitlab.com/CreepySkeleton/proc-macro-error