Avoid stack overflow when checking sealing on cyclic trait bounds.

[obi1kenobi]

Fixes #1076.

[obi1kenobi]

Proof of fix:

$ cargo run --release -- semver-checks --manifest-path /tmp/zproto/Cargo.toml 
    Finished `release` profile [optimized] target(s) in 0.23s
     Running `target/release/cargo-semver-checks semver-checks --manifest-path /tmp/zproto/Cargo.toml`
    Building zproto v0.4.1-pre (current)
       Built [   1.163s] (current)
     Parsing zproto v0.4.1-pre (current)
      Parsed [   0.059s] (current)
     Parsing zproto v0.4.0 (baseline, cached)
      Parsed [   0.061s] (baseline)
    Checking zproto v0.4.0 -> v0.4.1-pre (minor change)
     Checked [   0.080s] 121 checks: 119 pass, 2 fail, 0 warn, 13 skip

--- failure auto_trait_impl_removed: auto trait no longer implemented ---

Description:
A public type has stopped implementing one or more auto traits. This can break downstream code that depends on the traits being implemented.
        ref: https://doc.rust-lang.org/reference/special-types-and-traits.html#auto-traits
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.39.0/src/lints/auto_trait_impl_removed.ron

Failed in:
  type Mock is no longer Send, in /tmp/zproto/src/backend.rs:131
  type Mock is no longer Sync, in /tmp/zproto/src/backend.rs:131

--- failure struct_missing: pub struct removed or renamed ---

Description:
A publicly-visible struct cannot be imported by its prior path. A `pub use` may have been removed, or the struct itself may have been renamed or removed entirely.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#item-remove
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.39.0/src/lints/struct_missing.ron

Failed in:
  struct zproto::ascii::port::OpenSerialOptions, previously in file /home/predrag/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/zproto-0.4.0/src/ascii/port/options.rs:31
  struct zproto::ascii::port::OpenTcpOptions, previously in file /home/predrag/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/zproto-0.4.0/src/ascii/port/options.rs:171

     Summary semver requires new major version: 2 major and 0 minor checks failed
    Finished [   1.739s] zproto

whereas previously running this command resulted in a stack overflow and crash.