Allow double-quote and backslash in string and char literals

[julianhyde]

Allow double-quote and backslash in string and char literals. Per Standard ML syntax, string and char literals are enclosed in double-quotes, and double-quotes and backslash are escaped using a backslash.

Here are some examples (run in sml-nj v110.79):

- "a string literal with a \"quoted\" word";
val it = "a string literal with a \"quoted\" word" : string
- "a string with a backslash \\ and a forward slash /";
val it = "a string with a backslash \\ and a forward slash /" : string
- #"a";
val it = #"a" : char
- #"\"";
val it = #"\"" : char
- #"\\";
val it = #"\\" : char