KCVM instructions should include comment

[adamchalmers]

Background

KCVM is a low-level language, its instructions are usually generated by a compiler.

Problem

When debugging KCVM it's unclear why the high-level KCL generated a particular KCVM instruction.

Solution

Each Instruction should (optionally) support a comment string. That way compilers could explain "why" a particular instruction was included in the final instruction set.

Two suggested implementation approaches:

1. Adding a comment: Option<String> field to the Instruction type
2. Adding a map from instruction IDs to comment strings.

The KCVM debugger should show these comments.

Grackle should then add comments to the KCVM instructions it emits.