Float32 support

Looks like it is working:

#use "topfind";;
#require "biniou";;

let x = -1.250;;
let ob = Bi_outbuf.create 10;;
Bi_io.write_untagged_float32 ob x;
let x_str = Bi_outbuf.contents ob;;
let ib = Bi_inbuf.from_string x_str;;
let x' = Bi_io.read_untagged_float32 ib in
(x = x', x, x');;

I am just not sure if we should care about endianness.
I did not as the functions I am reusing do not:
read/write_untagged_int32.

Regards,
F.