Update MSRV to 1.57

Author: jturner314

.github/workflows/ci.yaml

@@ -34,7 +34,7 @@ jobs:
           - stable
           - beta
           - nightly
-          - 1.51.0  # MSRV
+          - 1.57.0  # MSRV
 
     name: tests/${{ matrix.rust }}
     steps:

Cargo.toml

@@ -3,7 +3,7 @@
 name = "ndarray"
 version = "0.15.6"
 edition = "2018"
-rust-version = "1.51"
+rust-version = "1.57"
 authors = [
   "Ulrik Sverdrup \"bluss\"",
   "Jim Turner"