Sometimes we just want to fix up the printed name of the instruction in
the disassembly; e.g., instructions with the `lock` prefix. But, as
we're seeing in bytecodealliance#10836 and elsewhere, other times we want to control the
printing of the entire instruction--the full `Display` implementation.
We'll need this kind of thing if we want to swap the order of operands
or omit an immediate if it ends up being included in the printed
instruction name.

This change splits this kind of customization in two: `Mnemonic` and
`Display`. After renaming this enum to `Customization`, all existing
uses of `Display` are migrated to `Mnemonic`.