Implement the TODOs in the Harvard Architecture PR

[matthiasgoergens]

In the Harvard Architecture PR we left a few TODOs. Here we make good on them.

Alas, unlike @icemelon's hopes the change is slightly more involved.

There's a few things happening in this PR:

• We remove the fake instruction EANY. EANY was only ever an artifact of Risc0's incredible 'interesting' approach to decoding. We use the real instructions ECALL and EBREAK instead.

• Remove AUIPC and LUI. Both of them are now implemented as pseudo-instructions, that translate to ADDI during decoding.

• Use the same library as SP1 for RiscV decoding, instead of copy-and-pasting-and-editing Risc0's 'interesting' decoder. That simplifies our code, and comes with a lot more tests than we ever had. Both because of explicit tests in the library, and because of the usage in SP1 and other projects. This gets rid of much error prone bit manipulating code.

• Use struct Instruction throughout the code when handling and testing instructions, instead of u32. That makes specifying tests a lot simpler and more readable. No more 0b_000000001010_00000_000_00001_0010011, // addi x1, x0, 10 in the code.

• Remove the notion of executable vs non-executable ROM. This is only necessary for a von-Neumann architecture: everything that's in our instruction-cache is meant to be executable already. (We can re-implement this restriction later by controlling what is allowed to make it into the instruction cache when we eg decode the ELF. But it's unnecessary: we already honour the executable flag for memory sections in the ELF.)

Please review with whitespace changes hidden for an easier reading experience.

[icemelon]

LGTM

[naure]

Remove AUIPC and LUI.

The LUI circuit is more efficient than ADDI. No register read, fewer range checks, no addition... I recommend putting it back.

You can implement AUIPC with the LUI circuit though (absolute addressing).

[matthiasgoergens]

If you think it's worth the hassle, we can restore the LUI circuit and also compile ADDI with rs1=0 to LUI.

[naure]

Indeed.