Add an "if" function

[lovasoa]

If the first argument is true, returns the second argument, otherwise, return the third

[lovasoa]

By the way, the signature of pub fn as_fixed_len_tuple(&self, len: usize) -> EvalexprResult<TupleType> is a little bit strange. It should probably be either

pub fn as_fixed_len_tuple(&self, len: usize) -> EvalexprResult<&TupleType>

or

pub fn into_fixed_len_tuple(self, len: usize) -> EvalexprResult<TupleType>

[lovasoa]

@ISibboI, what do you think ?

[ISibboI]

Thanks, looks good!

[lovasoa]

:tada: