Bump MSRV to 1.46.0 (#781)

Author: thomcc

.github/workflows/main.yml

@@ -117,7 +117,7 @@ jobs:
     steps:
     - uses: actions/checkout@master
     - name: Install Rust
-      run: rustup update 1.34.0 --no-self-update && rustup default 1.34.0
+      run: rustup update 1.46.0 --no-self-update && rustup default 1.46.0
       shell: bash
     - run: cargo build