Raise minimum tested compiler to 1.59

Required for const-initialized thread locals support.

    error[E0658]: use of unstable library feature 'thread_local_const_init'
     --> src/dedup.rs:3:1
      |
    3 | / thread_local! {
    4 | |     static REFCOUNT: Cell<usize> = const { Cell::new(0) };
    5 | | }
      | |_^
      |

Author: dtolnay

.github/workflows/ci.yml

@@ -65,7 +65,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        rust: [beta, stable, 1.56.0]
+        rust: [beta, stable, 1.59.0]
     timeout-minutes: 45
     steps:
       - uses: actions/checkout@v4