Add support for allocating statically-aligned and dynamically-sized memory

*See also: #885, #249, #280.*

**What is the name of your project?**

Fuchsia

**Please provide a link to your project (GitHub repository, crates.io page, etc).**

https://cs.opensource.google/fuchsia/fuchsia/+/main:src/storage/fvm/src/main.rs;drc=b04c5108ca31a3a8880119d06dad7e857c2a9b12;l=1598

**What features would you like from zerocopy?**

We have a need to allocate memory that has the same alignment as some other type T (that has the zerocopy traits) but is larger than T (it's used for storage so it needs to be rounded to block sizes). Once that's memory as been allocated, ideally it would be possible to use infallible zerocopy methods since the alignment (it's aligned) and size (it's big enough) should be known to be valid.