Features to support EMP

**What is the name of your project?**: Encrypted Mesh Protocol

**Please provide a link to your project (GitHub repository, crates.io page, etc).**: paper is currently being written alongside a rust reference implementation. neither is yet publicly published

**What features would you like from zerocopy?**
For each feature:
- I would like to be able to define the length of a DST out of band of the slice, that way I can use preceding metadata in a network packet to specify the bounds of multiple dynamically sized memory locations

see prior discussion here: https://discord.com/channels/1246168296248578128/1246168297549074455/1291859740766502983