chore: set msrv to 1.57.0 (result of cargo msrv) (#322)

* Add rust MSRV to CI tests
* Run clippy on stable + MSRV
* Run 32 bit tests on MSRV

Skip tesking MSRV w/valgrind due to issues
installing cargo-valgrind. To do this,
we'd need to figure out a specific version
of that tool.

Closes #321

Author: molpopgen

.github/workflows/test32bit.yml

@@ -15,6 +15,7 @@ jobs:
         os: [ubuntu-latest]
         rust:
           - stable
+          - 1.57.0
     steps:
       - name: Cancel Previous Runs
         uses: styfle/cancel-workflow-action@0.6.0

.github/workflows/tests.yml

@@ -15,6 +15,7 @@ jobs:
         os: [ubuntu-latest, macos-latest]
         rust:
           - stable
+          - 1.57.0
     steps:
       - name: Cancel Previous Runs
         uses: styfle/cancel-workflow-action@0.6.0
@@ -62,12 +63,17 @@ jobs:
   clippy:
     name: clippy!!
     runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        rust:
+          - stable
+          - 1.57.0
     steps:
       - uses: actions/checkout@v2
       - uses: actions-rs/toolchain@v1
         with:
           profile: minimal
-          toolchain: stable
+          toolchain: ${{ matrix.rust }}
           override: true
       - uses: Swatinem/rust-cache@v1
       - run: rustup component add clippy

Cargo.toml

@@ -8,6 +8,7 @@ description = "rust interface to tskit"
 license = "MIT"
 homepage = "https://github.com/tskit-dev/tskit-rust"
 repository = "https://github.com/tskit-dev/tskit-rust"
+rust-version = "1.57.0"
 
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html