perf: Shrink MemSliceInner enum

[nameexhaustion]

Learned a few things after looking into the bytes::Bytes implementation. They essentially already do most of what we want from MemSliceInner, just without the Mmap support:

• Creating a Bytes from a Vec is zero-copy
• Creating an empty Bytes::new() doesn't heap-allocate
• They have const fn Bytes::from_static we can use for the &'static [u8] case, which also doesn't allocate

This shrinks the size of MemSliceInner from 40 to 32 bytes