Fix website

The arc-script online REPL currently fails to compile with the error:

    Exported global cannot be mutable

It is a known error:

    rustwasm/wasm-pack#886

This commit is a fix for the error + a small cleanup of unused code.

arc-script/wasm/Cargo.toml

@@ -21,3 +21,6 @@ wasm-bindgen-test = "0.2"
 
 [profile.release]
 opt-level = "s"
+
+[package.metadata.wasm-pack.profile.release]
+wasm-opt = false

arc-script/wasm/src/lib.rs

@@ -1,6 +1,5 @@
 mod utils;
 
-use arc_script::pretty::Pretty;
 use arc_script::opt::*;
 use wasm_bindgen::prelude::*;