Fixes #8: completely rewrote instruction decoding and execution, handle compressed natively

The decoder is generated by gendecoder; run
```
cargo r -p gendecoder --bin gendecoder > tmp;mv tmp src/generated_riscv_decoder.rs
```
when adding new instructions.  (We skipped build.rs as changes are
expected to be very rare).

Also hundreds of tiny adjustments and improvements

Author: tommythorn

Cargo.toml

@@ -15,7 +15,7 @@ exclude = [
 
 [workspace]
 resolver = "3"
-members = [".", "sim", "wasm"]
+members = [".", "sim", "gendecoder", "wasm"]
 default-members = ["sim"]
 
 [lints.clippy]

gendecoder/Cargo.toml

@@ -0,0 +1,13 @@
+[package]
+name = "gendecoder"
+version = "0.1.0"
+edition = "2024"
+
+[dependencies]
+anyhow = "1.0.100"
+argh = "0.1.13"
+env_logger = "0.11.8"
+libc = "0.2.176"
+simmerv = { path = ".." }
+termios = "0.3.3"
+zstd = "0.13.3"

gendecoder/src/main.rs

@@ -0,0 +1,7 @@
+use simmerv::riscv_insns::generate_riscv_decoder;
+
+fn main() -> anyhow::Result<()> {
+    generate_riscv_decoder();
+
+    Ok(())
+}