Add no_std support, by adding an std feature (#281)

To make the library work as `no_std` we add an `std` feature which
is on by default. When it is off, we compile as `no_std` and make
parts of the API that require `std::io` conditional on the `std`
feature.

ci/azure-test-stable.yml

@@ -40,3 +40,7 @@ jobs:
   - ${{ if parameters.benches }}:
     - script: cargo check --benches
       displayName: Check benchmarks
+
+  # Run with no default defaults
+  - script: cargo ${{ parameters.cmd }}
+    displayName: cargo ${{ parameters.cmd }} --no-default-features