Request `in` operator

[rebcabin]

I'll work around it for now, but the in operator would be useful. Here is part of my hand-written lexer that could use it:

def eat_white(self : LasrLexer) -> None:
    c: str = peek(self)
    while c != EOF_SIGNAL and c in WHITESPACE:
        c = read(self.fd, 1)
        if c == NEWLINE:
            self.col = 1
            self.line += 1
        else:
            self.col += 1
        c = peek(self)

produces:

/Users/brian/CLionProjects/lpython/src/bin/python /Users/brian/CLionProjects/lpython/lasr/LP-pycharm/lasr_lexer.py 
semantic error: Comparison operator not implemented
   --> /Users/brian/CLionProjects/lpython/lasr/LP-pycharm/lasr_lexer.py:163:31
    |
163 |     while c != EOF_SIGNAL and c in WHITESPACE:
    |                               ^^^^^^^^^^^^^^^ 

[rebcabin]

My work around is not bad and might be a good example in a programmer's manual if we decide not to implement the in operator:

def in_(c: str, l: list[str]) -> bool:
    result : bool = False
    s : str
    for s in l:
        if s == c:
            result = True
            break
    return result


def eat_white(self : InOut[LasrLexer]) -> None:
    c: str = peek(self)
    while c != EOF_SIGNAL and in_(c, WHITESPACE):
        c = read(self.fd, 1)
        if c == NEWLINE:
            self.col = 1
            self.line += 1
        else:
            self.col += 1
        c = peek(self)

[certik]

I think we should implement in.