Refactor to split `Opcode` by instruction format

Currently there's one `Opcode` enum which defines all the opcodes. This means it's possible to create nonsense instructions that should not ever appear in RISC-V code, such as opcodes for the wrong instruction format.

This should be split apart so each instruction format has a separate opcode enum, to get better type safety.

Start by locating the existing Opcode enum and the instruction-format definitions in the repository, then inspect how instructions are constructed and validated. Done means each instruction format uses its own opcode enum so invalid opcode and format combinations cannot be represented; verify this with the project's existing checks.

Written by the indexing model from the issue text.