Function encode_integer 

pub fn encode_integer(
    value: u64,
    prefix_bits: u8,
    out_byte_prefix_bits: u8,
) -> Vec<u8> 

Encode value with a prefix_bits-bit prefix slot. Spec §5.1.

prefix_bits must be in 1..=8. The first output byte contains the (8 - prefix_bits) most-significant bits from out_byte_prefix_bits (the caller supplies them in bits 7..prefix_bits).