[Feature]: Include function signatures in operation elements

The operation elements publish `Instr_*` function bodies but not their parameter types or return types. The bodies use free variables whose types aren't stated, and the call sites in the iform code pass untyped expressions.

This is a large obstacle to turning the XML into a checkable codebase and impairs readability (the rendered operation code contains names whose types and widths are never stated).

Emitting the function header as part of the operation element would resolve this.

Start by tracing how operation elements publish the `Instr_*` function bodies and how the iform code supplies their call-site expressions. Determine how function headers can be emitted alongside those bodies. Done means each operation element includes parameter types and return types so free variables and call sites are explicitly typed.

Written by the indexing model from the issue text.