ASR: Keep strings unescaped in AST, ASR

[Shaikh-Ubaid]

fixes #1759.

[Shaikh-Ubaid]

Example:

$ cat examples/expr2.py
def main0():
    print(r"\n\t\r\
            \n\t\
          hi")

    print('I said \'Wow\' ')
    print("I said \"hi\" ")
    print("I said \\\"hi\" ")
    print("abc \\ ")
    print("abc \\\ wow")
    print('I said \'\\\'hi\\''hey\' ')
    print('  ''hey\' ')
    print(" " " hey\" ")

main0()
$ python examples/expr2.py 
\n\t\r\
            \n\t\
          hi
I said 'Wow' 
I said "hi" 
I said \"hi" 
abc \ 
abc \\ wow
I said '\'hi\hey' 
  hey' 
  hey" 
$ lpython examples/expr2.py
\n\t\r\
            \n\t\
          hi
I said 'Wow' 
I said "hi" 
I said \"hi" 
abc \ 
abc \\ wow
I said '\'hi\hey' 
  hey' 
  hey" 

[certik]

Thanks for fixing this!