encode() and decode() both have failure modes if you pass in a source or destination buffer that is too small but the return type is usize for both encode() and decode().

The _with_offset() routines could likewise be removed, since slices in rust are very cheap.

These are the routines I'm using inline in desert but I considered using this varinteger first:

https://github.com/substack/desert/blob/46789f4af0e98afb80438cb80db573d0d8dbb630/src/lib.rs#L1020-L1054