CI: Test MSRV

There are multiple MSRVs in this crate because of the `bitcoin_hashes`
dependency and also the `zeroize` feature.

Add MSRV test jobs for the three MSRVs effected by `bitcoin_hashes`
and also one for the `zeroize` feature.

Includes committing a lock file for Rust 1.41 and 1.48, the latter
works for all other MSRV versions tested.

Check and fix the docs in the README while we are at it.

Author: tcharding

.github/workflows/rust.yml

@@ -50,7 +50,7 @@ jobs:
           DO_NO_STD: true
         run: ./contrib/test.sh
 
-  MSRV:
+  MSRV-1-41:
     name: Test - 1.41.1 toolchain
     runs-on: ubuntu-latest
     strategy:
@@ -66,6 +66,54 @@ jobs:
           DO_FEATURE_MATRIX: true
         run: ./contrib/test.sh
 
+  MSRV-1-48:
+    name: Test - 1.48.0 toolchain
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+    steps:
+      - name: Checkout Crate
+        uses: actions/checkout@v3
+      - name: Checkout Toolchain
+        uses: dtolnay/[email protected]
+      - name: Running test script
+        env:
+          DO_NO_STD: true
+          DO_FEATURE_MATRIX: true
+        run: ./contrib/test.sh
+
+  MSRV-1-51:
+    name: Test - 1.51.0 toolchain
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+    steps:
+      - name: Checkout Crate
+        uses: actions/checkout@v3
+      - name: Checkout Toolchain
+        uses: dtolnay/[email protected]
+      - name: Running test script
+        env:
+          DO_NO_STD: true
+          DO_FEATURE_MATRIX: true
+        run: ./contrib/test.sh
+
+  MSRV-1-56:
+    name: Test - 1.56.0 toolchain
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+    steps:
+      - name: Checkout Crate
+        uses: actions/checkout@v3
+      - name: Checkout Toolchain
+        uses: dtolnay/[email protected]
+      - name: Running test script
+        env:
+          DO_NO_STD: true
+          DO_FEATURE_MATRIX: true
+        run: ./contrib/test.sh
+
   Arch32bit:
     name: Test 32-bit version
     runs-on: ubuntu-latest

Cargo-minimal.lock renamed to Cargo-msrv-1.41.lock

@@ -0,0 +1,328 @@
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+# It is not intended for manual editing.
+version = 3
+
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