Add support for character enclosed within single quotes. Single
character without quotes would work for most characters, but not
'#' and possibly not '^' or '\\'. All the values in dat/symbols
are specified via backslash+digits so it isn't obvious that some
other form of value is allowed.
I think this parsing accepts all valid values. It doesn't reject
all invalid ones: opening quote followed by valid escape sequence
followed by junk followed by closing quote will ignore the junk.
I don't think there's any pressing need to worry about that.
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