Add decoder for C extension

[tandr3w]

Issue #239

Implemented decompression for all rv32 instructions other than F and D extension ones

Also added checking for illegal/reserved and unimplemented instructions though I don't think we have a way to handle that properly yet?

Note C.JAL and C.JALR need extra handling to work properly (I think)

[github-actions]

🔧 DE1-SoC Synthesis Report Summary Diff

• RV32I

  1. Fitter Summary

     @@ -1,4 +1,4 @@
     -Fitter Status : Successful - Wed Jul  8 03:40:10 2026
     +Fitter Status : Successful - Fri Jul 10 02:17:12 2026
      Quartus Prime Version : 25.1std.0 Build 1129 10/21/2025 SC Lite Edition
      Revision Name : utoss-risc-v
      Top-level Entity Name : top

  2. Fitter by entity
     No changes detected

  3. Timing
     No changes detected

• RV32IB

  1. Fitter Summary

     @@ -1,4 +1,4 @@
     -Fitter Status : Successful - Wed Jul  8 03:40:01 2026
     +Fitter Status : Successful - Fri Jul 10 02:17:14 2026
      Quartus Prime Version : 25.1std.0 Build 1129 10/21/2025 SC Lite Edition
      Revision Name : utoss-risc-v
      Top-level Entity Name : top

  2. Fitter by entity
     No changes detected

  3. Timing
     No changes detected

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Comparing synthesis results from main branch vs. this PR