Generate code to:
- Unwrap the instruction and generate an error if the instruction format
doesn't match the recipe.
- Look up the value locations of register and stack arguments.
The recipe_* functions in the ISA binemit modules now take these
unwrapped items as arguments.
Also add an optional `emit` argument to the EncRecipe constructor which
makes it possible to provide inline Rust code snippets for code
emission. This requires a lot less boilerplate than recipe_* functions.
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