class input_string :Creates an input channel from a (constant) string.
?pos:int -> ?len:int -> string ->
pos_in reflects the real position in the string, i.e.
a character read at position
k can be found at
s.[k] in the string
pos: The data of the channel begins at this position of the string. Default: 0
len: The data of the channel consists of this number of bytes. Default: until the end of the string