Bump MSRV to 1.56.1

We want to depend on the newly released version of `hashes` which has an
MSRV of 1.56.1

Bump the MSRV for the main crate `secp256k1` to Rust `v1.56.1` as it is
for most of the rest of our crates. Leave `secp256k1-sys` as it is, and
add a basic CI job to test we can run tests with default features using
1.48.0 still for that crate only.

.github/workflows/rust.yml

@@ -54,8 +54,8 @@ jobs:
           DO_ASAN: true
         run: ./contrib/test.sh
 
-  MSRV:
-    name: Test - 1.48.0 toolchain
+  MSRV-secp256k1-sys:
+    name: secp256k1-sys - test 1.48.0
     runs-on: ubuntu-latest
     strategy:
       fail-fast: false
@@ -64,6 +64,19 @@ jobs:
         uses: actions/checkout@v3
       - name: Checkout Toolchain
         uses: dtolnay/rust-toolchain@1.48.0
+      - name: Test default features
+        run: cd secp256k1-sys && cargo test
+
+  MSRV:
+    name: Test - 1.56.1 toolchain
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+    steps:
+      - name: Checkout Crate
+        uses: actions/checkout@v3
+      - name: Checkout Toolchain
+        uses: dtolnay/rust-toolchain@1.56.1
       - name: Running test script
         env:
           DO_FEATURE_MATRIX: true

Cargo.toml

@@ -10,7 +10,8 @@ documentation = "https://docs.rs/secp256k1/"
 description = "Rust wrapper library for Pieter Wuille's `libsecp256k1`. Implements ECDSA and BIP 340 signatures for the SECG elliptic curve group secp256k1 and related utilities."
 keywords = [ "crypto", "ECDSA", "secp256k1", "libsecp256k1", "bitcoin" ]
 readme = "README.md"
-edition = "2018"
+edition = "2021"
+rust-version = "1.56.1"
 
 [package.metadata.docs.rs]
 all-features = true

README.md

@@ -29,7 +29,7 @@ Contributions to this library are welcome. A few guidelines:
 * Any breaking changes must have an accompanied entry in CHANGELOG.md
 * No new dependencies, please.
 * No crypto should be implemented in Rust, with the possible exception of hash functions. Cryptographic contributions should be directed upstream to libsecp256k1.
-* This library should always compile with any combination of features on **Rust 1.48.0**.
+* This library should always compile with any combination of features on **Rust 1.56.1**.
 
 ### Githooks