Cargo.toml: specify minimum rust version

This field causes cargo to automatically verify that a recent enough
rustc version is being used. For cargo versions before 1.56, adding this
option creates a build warning - from [0]:

> The first version of Cargo that supports this field was released with Rust
> 1.56.0. In older releases, the field will be ignored, and Cargo will display
> a warning.

[0]: https://doc.rust-lang.org/cargo/reference/manifest.html#the-rust-version-field

Cargo.toml

@@ -13,6 +13,9 @@ categories = ["parsing"]
 edition = "2018"
 autoexamples = false
 
+# also update in README.md (badge and "Rust version requirements" section)
+rust-version = "1.48"
+
 include = [
   "CHANGELOG.md",
   "LICENSE",