Base64

`sig
exception Invalid_char
exception Invalid_table
type encoding_table = char array
type decoding_table = int array
val str_encode : ?tbl:Base64.encoding_table -> string -> string
val str_decode : ?tbl:Base64.decoding_table -> string -> string
val encode : ?tbl:Base64.encoding_table -> 'a IO.output -> 'a IO.output
val decode : ?tbl:Base64.decoding_table -> IO.input -> IO.input
val make_decoding_table : Base64.encoding_table -> Base64.decoding_table
end`