Remove recursion from character escape handling in reader

Remove recursion from character escape handling in reader

This cures a C stack overflow when reading certain long (crafted)
strings (bug#62039) and improves performance of reading escaped
characters in character and string literals.
Reported by Bruno Haible.

* src/lread.c (invalid_escape_syntax_error): New.
(read_escape): Rename to...
(read_char_escape): ...this.  Remove recursion.  Pass read-ahead char
as argument.  Improve code performance and clarity.
(read_char_literal, read_string_literal): Update calls.
* test/src/lread-tests.el (lread-char-modifiers)
(lread-many-modifiers): Add test cases.