Failure to compile with Rust 1.75

[akonradi-signal]

When trying to use the new version with the Rust 1.75 compiler, I get the following error:

error[E0658]: use of unstable library feature 'error_in_core'
  --> arbitrary-1.4.0/src/lib.rs:52:6
   |
52 | impl core::error::Error for MaxRecursionReached {}

It looks like while std::error::Error has been around for a while, its presence in core is new as of Rust 1.81

[suikammd]

Same. Incompatible upgrade, should release major version.

[akonradi-signal]

It seems like this was an oversight. Fixing as in #204 and releasing a 1.4.1 seems like the right way to move forward.