Declare MSRV 1.58

Author: sourcefrog

Cargo.toml

@@ -8,6 +8,7 @@ description = "Find inadequately-tested code that can be removed without any tes
 repository = "https://github.com/sourcefrog/cargo-mutants"
 categories = ["development-tools::testing"]
 keywords = ["testing", "mutants", "cargo", "mutation-testing", "coverage"]
+rust-version = "1.58"
 
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html