Code action: Change "base" for character/string literals

[Kmeakin]

Just as how integer literals have a "Convert integer base" code action for converting between decimal/binary/octal/hexadecimal, there should be a code action to convert between the different ways of writing character literals and characters in string literals:

• Normal character (eg 'a')
• ASCII escape (eg '\x61')
• Unicode escape (eg '\u{61}')
• C-style escapes, for '\n', '\r', '\t', '\\', '\0', '\'', '\"'

[Young-Flash]

@rustbot claim