Fix build on non-32/64-bit architectures (#1144)

* Impl WideningMultiply for 16-bit pointers
* Add AVR CI test job

.github/workflows/test.yml

@@ -174,6 +174,20 @@ jobs:
       - name: Build top-level only
         run: cargo build --target=thumbv6m-none-eabi --no-default-features
 
+  test-avr:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - name: Install toolchain
+        uses: actions-rs/toolchain@v1
+        with:
+            profile: minimal
+            toolchain: nightly-2021-01-07 # Pinned compiler version due to https://github.com/rust-lang/compiler-builtins/issues/400
+            components: rust-src
+            override: true
+      - name: Build top-level only
+        run: cargo build -Z build-std=core --target=avr-unknown-gnu-atmega328 --no-default-features
+
   test-ios:
     runs-on: macos-latest
     steps:

src/distributions/utils.rs

@@ -138,6 +138,8 @@ macro_rules! wmul_impl_usize {
         }
     };
 }
+#[cfg(target_pointer_width = "16")]
+wmul_impl_usize! { u16 }
 #[cfg(target_pointer_width = "32")]
 wmul_impl_usize! { u32 }
 #[cfg(target_pointer_width = "64")]