Deny warnings on CI (#1372)

## Issue Addressed

Prevent CI from succeeding when there are warnings. Code can still be built and tested with warnings locally, but CI may fail during a Rust update (which is fine IMO).

## Proposed Changes

* Deny warnings for all stable compiler jobs on CI (excludes `cargo udeps`, which runs under nightly)
* Fix the warnings currently on `master` related to unnecessary `mem::replace`

.github/workflows/test-suite.yml

@@ -8,7 +8,9 @@ on:
       - trying
       - 'pr/*'
   pull_request:
-
+env:
+  # Deny warnings in CI
+  RUSTFLAGS: "-D warnings"
 jobs:
   cargo-fmt:
     name: cargo-fmt
@@ -149,3 +151,6 @@ jobs:
       run: cargo install cargo-udeps --locked
     - name: Run cargo udeps to identify unused crates in the dependency graph
       run: make udeps
+    env:
+      # Allow warnings on Nightly
+      RUSTFLAGS: ""