Compact U64

A machine-friendly varint, implemented in Rust.

The general idea is the following:

• Each encoding is preceeded by a tag of two to eight (inclusive) bits.
• Each u64 can be encoded by setting the tag to the greatest possible number and then encoding the u64 as an eight-byte big-endian integer.
• Each u64 that fits into four bytes can be encoded by setting the tag to the second-greatest possible number and then encoding the u64 as an four-byte big-endian integer.
• Each u64 that fits into two bytes can be encoded by setting the tag to the third-greatest possible number and then encoding the u64 as an two-byte big-endian integer.
• Each u64 that fits into one byte can be encoded by setting the tag to the fourth-greatest possible number and then encoding the u64 as an one-byte big-endian integer.
• If the tag has more than two bits, then each u64 that is less than the fourth-greatest tag can be encoded in the tag directly, followed by no further bytes.