Inline dyn

A relatively simple example wrapper working (kinda) as a by-value dyn Trait.

Limitations:

• The wrapper size is fixed. The content either fits, or it doesn’t compile.
• Supporting each trait requires a bit of code, as there is no way to express “for each trait X” in Rust.
• Requires unstable features (notably ptr_metadata, others are probably avoidable).
• unsafe here, unsafe there, unsafe everywhere! (It does pass Miri though).