bug#55738: character escape bugs in the reader

[Lars Ingebrigtsen <larsi@gnus.org>]

Mattias Engdegård <mattiase@acm.org> writes:

> Make the character literal ?\LF (linefeed) generate 10, not -1.
>
> Ensure that Control escape sequences in character literals are
> idempotent: ?\C-\C-a and ?\^\^a mean the same thing as ?\C-a and ?\^a,
> generating the control character with value 1.  "\C-\C-a" no longer
> signals an error.

I think both changes make sense.

> * src/lread.c (read_escape): Make nonrecursive and only combine
> the base char with modifiers at the end, creating control chars
> if applicable.  Remove the `stringp` argument; assume character
> literal syntax.  Never return -1.
> (read_string_literal): Handle string-specific escape semantics here
> and simplify.

And also sounds good.

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