Update MSRV to 1.56.1

Following `rust-bitcoin` MSRV of 1.56.1, this pins MSRV to 1.56.1.

Quoting @tcharding
in rust-bitcoin/rust-bitcoin@d9cc724

> Rust version 1.56.0 introduced edition 2021. Shortly afterwards, on
> October 21 2021 Rust version 1.56.1 was released.

> Debian stable is currently shipping `rustc 1.63.0`.

> Our stated MSRV policy is: In Debian stable and at least 2 years old.

> Therefore our MSRV policy is met by Rust version 1.56.1 and we can strat
> to bump our MSRV org wide.

> Links:
> - https://blog.rust-lang.org/2021/11/01/Rust-1.56.1.html
> - https://blog.rust-lang.org/2021/10/21/Rust-1.56.0.html
> - https://packages.debian.org/stable/rust/rustc

.github/workflows/rust.yml

@@ -54,7 +54,7 @@ jobs:
           - rust: stable
           - rust: beta
           - rust: nightly
-          - rust: 1.48
+          - rust: "1.56.1"
     steps:
       - name: Checkout Crate
         uses: actions/checkout@v2

README.md

@@ -1,6 +1,6 @@
 ![Build](https://github.com/rust-bitcoin/rust-miniscript/workflows/Continuous%20integration/badge.svg)
 
-**Minimum Supported Rust Version:** 1.48.0
+**Minimum Supported Rust Version:** 1.56.1
 
 # Miniscript
 
@@ -40,7 +40,8 @@ The cargo feature `std` is enabled by default. At least one of the features `std
 Enabling the `no-std` feature does not disable `std`. To disable the `std` feature you must disable default features. The `no-std` feature only enables additional features required for this crate to be usable without `std`. Both can be enabled without conflict.
 
 ## Minimum Supported Rust Version (MSRV)
-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**.
 
 Some dependencies do not play nicely with our MSRV, if you are running the tests
 you may need to pin some dependencies. See `./contrib/test.sh` for current pinning.

clippy.toml

@@ -1,3 +1,3 @@
-msrv = "1.48.0"
+msrv = "1.56.1"
 # plan API returns Self as an error type for an large-ish enum
 large-error-threshold = 256

contrib/test.sh

@@ -20,12 +20,12 @@ then
     cargo fmt -- --check
 fi
 
-# Pin dependencies required to build with Rust 1.48.0
-if cargo --version | grep "1\.48\.0"; then
+# Pin dependencies required to build with Rust 1.56.1
+if cargo --version | grep "1\.56\.1"; then
     cargo update -p once_cell --precise 1.13.1
-    cargo update -p quote --precise 1.0.28
-    cargo update -p syn --precise 2.0.32
-    cargo update -p proc-macro2 --precise 1.0.63
+    cargo update -p quote --precise 1.0.35
+    cargo update -p syn --precise 2.0.46
+    cargo update -p proc-macro2 --precise 1.0.74
     cargo update -p serde_json --precise 1.0.99
     cargo update -p serde --precise 1.0.152
     cargo update -p log --precise 0.4.18