fix python binding for concat, concat_ws, and random

[jimexist]

Which issue does this PR close?

fix python binding for concat, concat_ws, and random

Closes #226

Rationale for this change

What changes are included in this PR?

Are there any user-facing changes?

>>> import datafusion
>>> from datafusion import functions as f
>>> dir(f)
['__all__', '__doc__', '__loader__', '__name__', '__package__', '__spec__', 'abs', 'acos', 'array', 'ascii', 'asin', 'atan', 'avg', 'bit_length', 'btrim', 'ceil', 'character_length', 'chr', 'col', 'concat', 'concat_ws', 'cos', 'count', 'exp', 'floor', 'in_list', 'initcap', 'left', 'lit', 'ln', 'log10', 'log2', 'lower', 'lpad', 'ltrim', 'max', 'md5', 'min', 'now', 'octet_length', 'random', 'regexp_replace', 'repeat', 'replace', 'reverse', 'right', 'round', 'rpad', 'rtrim', 'sha224', 'sha256', 'sha384', 'sha512', 'signum', 'sin', 'split_part', 'sqrt', 'starts_with', 'strpos', 'substr', 'sum', 'tan', 'to_hex', 'translate', 'trim', 'trunc', 'udaf', 'udf', 'upper']
>>> f.concat(f.lit(1), f.lit(2))
<builtins.Expression object at 0x1026fea30>
>>> f.concat(f.lit(1), f.lit(2), f.lit(3))
<builtins.Expression object at 0x1026feb70>

[alamb]

Thanks @jimexist -- I wonder if it would be helpful in the future to add a python level test for concat and concat_ws