Function preserves::value::writer::varint

pub fn varint<W: Write>(w: &mut W, v: u64) -> Result<usize>

Writes a varint to w. Returns the number of bytes written.

varint(n) = [n]                                  if n < 128
            [(n & 127) | 128] ++ varint(n >> 7)  if n ≥ 128