Update MSRV to Rust 1.64, matching ndarray 0.16

Author: bluss

.github/workflows/ci.yml

@@ -20,7 +20,7 @@ jobs:
           - stable
           - beta
           - nightly
-          - 1.49.0  # MSRV
+          - 1.64.0  # MSRV
     steps:
       - uses: actions/checkout@v2
       - uses: actions-rs/toolchain@v1