Escape whitespace and control characters in literals

Rewrites a String or char literal that spells out a control character, a tabulation or an invisible Unicode whitespace character so that it is written as \t, \b, \f or \uXXXX. Only the source spelling changes, so the value and the compiled class stay the same. In a text block every character is written as \uXXXX, and the incidental indentation and trailing whitespace the compiler strips are left alone, as are Kotlin-generated annotations and literals that escape a surrogate pair. Skips test sources.