Bump MSRV to 1.40

As discussed in commit 1a18345 ("Update dependenies", 2020-12-17), Rust
1.38's Cargo produces a backwards incompatible lock file format [1],
making "cargo update" effectively not work. cargo-lock [2] can change
the format of a lock file but that really doesn't fit into normal
operation. Bump MSRV to eliminate this hurdle; and move all the way to
1.40 to also eliminate the platform MSRV split.

[1] rust-lang/cargo#7579
[2] https://github.com/rustsec/cargo-lock

.github/workflows/ci.yml

@@ -22,7 +22,7 @@ jobs:
         include:
           - build: pinned-linux
             os: ubuntu-18.04
-            rust: 1.36.0
+            rust: 1.40.0
 
           - build: stable
             os: ubuntu-18.04