Returns a complex which denotes the string form. The parser ignores
leading whitespaces and trailing garbage. Any digit sequences can be
separated by an underscore. Returns zero for null or garbage string.
'9'.to_c #=> (9+0i)
'2.5'.to_c #=> (2.5+0i)
'2.5/1'.to_c #=> ((5/2)+0i)
'-3/2'.to_c #=> ((-3/2)+0i)
'-i'.to_c #=> (0-1i)
'45i'.to_c #=> (0+45i)
'3-4i'.to_c #=> (3-4i)
'-4e2-4e-2i'.to_c #=> (-400.0-0.04i)
'-0.0-0.0i'.to_c #=> (-0.0-0.0i)
'1/2+3/4i'.to_c #=> ((1/2)+(3/4)*i)
'ruby'.to_c #=> (0+0i)
See Kernel.Complex.
