…ation

Because `..` is otherwise not available to `cargo package`.

       Compiling proc-macro2-fallback v1.0.66
    error: couldn't read ../src/lib.rs: No such file or directory (os error 2)

    error: could not compile `proc-macro2-fallback` (lib) due to previous error
    error: failed to verify package tarball

Add proc-macro2-fallback crate containing just the fallback implemention

rust-lang/rust#113941

Fixed in nightly-2023-07-23.

This reverts commit c71dd29.

rust-fuzz/libfuzzer#105

Update syn 1.0 links to syn 2.0

Re-enable honggfuzz CI

    warning: unsafe function's docs miss `# Safety` section
        --> src/lib.rs:1240:5
         |
    1240 |     pub unsafe fn from_str_unchecked(repr: &str) -> Self {
         |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
         |
         = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#missing_safety_doc
         = note: `-W clippy::missing-safety-doc` implied by `-W clippy::all`
         = help: to override `-W clippy::all` add `#[allow(clippy::missing_safety_doc)]`

Parse rustc's representation of macro expansion error