Replace some usages of the old `unescape_` functions in AST, clippy and tests.
Replace some usages of the old `unescape_` functions in AST, clippy and tests.

graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust