Use miette for reporting errors

This gives us errors with annotated context like this:

```
Error:
  × type error: Unknown variable 'x'
    ╭─[isle_examples/let.isle:24:1]
 24 │   (Lower (B.B z))
 25 │   (A.Add x y))
    ·          ┬
    ·          ╰── Unknown variable 'x'
    ╰────
```

cranelift/isle/islec/Cargo.toml

@@ -10,3 +10,4 @@ log = "0.4"
 isle = { version = "*", path = "../isle/" }
 env_logger = { version = "0.8", default-features = false }
 clap = "2.33"
+miette = { version = "3.0.0", features = ["fancy"] }