Add implementation of jr instruction

[mrmr1993]

This PR implements the jr instruction. This builds on top of #1394.

[dannywillems]

I am a bit annoying, but while running programs in the VM, I liked seeing instructions decoded, like

debug!("Instr: jr {}", Env::debug_register(&rs));

Is it something you'd agree to add for each instruction?
In addition to some documentation for each?
Can be smth like:

            // lw: R[rt] <- M[R[rs] + sign_extended_imm]
            //                -------------------------
            //                         address
            //              ----------------------------
            //                        value
            ```
            
The helpers are in the last PRs I opened :). Might be useful for you to debug too.

[dannywillems]

Also, do you have any documentation regarding this next IP?

[mrmr1993]

Do you have any documentation regarding this next IP?

https://www.cs.cmu.edu/afs/cs/academic/class/15740-f97/public/doc/mips-isa.pdf

More importantly, it's what cannon does, and so we should match it if we want a proven version of cannon's execution.

[dannywillems]

Note: addresses must be aligned