Ast.unescape should not exist

Every time I come across this function I wonder why it exists. We use it to handle things like `"\n"` becoming an actual newline character, as opposed to backslash plus n. What I don't understand is why we don't do this in the lexer directly, because its whole job is to lex our string anyway.

I might be missing something though...

Start by locating Ast.unescape and the lexer code that handles string literals. Compare their responsibilities and existing escape handling; done means the project has a decided approach for where unescaping belongs, with behavior preserved and appropriate tests updated.

Written by the indexing model from the issue text.