Extract unescape from rustc_lexer into its own crate

Author: GuillaumeGomez

literal-escaper/Cargo.toml

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+[package]
+name = "literal-escaper"
+version = "0.0.0"
+edition = "2021"
+
+[dependencies]
+std = { version = '1.0.0', optional = true, package = 'rustc-std-workspace-std' }
+
+[features]
+rustc-dep-of-std = ["dep:std"]

literal-escaper/README.md

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+# literal-escaper
+
+This crate provides code to unescape string literals. It is used by `rustc_lexer`
+and `proc-macro`.