clean up ClosureSearcher api with hrtb

see
- https://stackoverflow.com/questions/35592750/how-does-for-syntax-differ-from-a-regular-lifetime-bound/35595491#35595491
- https://doc.rust-lang.org/nomicon/hrtb.html

tests/tests.rs

@@ -1,4 +1,4 @@
-use rlua::{Context, Lua, Table, UserData, UserDataMethods, Value};
+use rlua::{Context, Function, Lua, Table, UserData, UserDataMethods, Value};
 use rlua_searcher::{AddSearcher, Result};
 use std::collections::HashMap;
 use std::fs::File;
@@ -166,10 +166,15 @@ fn add_closure_searcher_works() {
 
     let mut modules: HashMap<
         &'static str,
-        Box<dyn Fn(Context) -> rlua::Result<&'static str> + Send>,
+        Box<
+            dyn for<'ctx> Fn(Context<'ctx>, Table<'ctx>, &str) -> rlua::Result<Function<'ctx>>
+                + Send,
+        >,
     > = HashMap::new();
 
-    let instrument_loader = Box::new(|lua_ctx: Context| {
+    let instrument_loader: Box<
+        dyn for<'ctx> Fn(Context<'ctx>, Table<'ctx>, &str) -> rlua::Result<Function<'ctx>> + Send,
+    > = Box::new(|lua_ctx, env, name| {
         let globals = lua_ctx.globals();
         let new = lua_ctx.create_function(|_, (name, sound): (String, String)| {
             Ok(Instrument::new(name, sound))
@@ -177,7 +182,11 @@ fn add_closure_searcher_works() {
         let tbl = lua_ctx.create_table()?;
         tbl.set("new", new)?;
         globals.set("instrument", tbl)?;
-        Ok("return instrument")
+        Ok(lua_ctx
+            .load("return instrument")
+            .set_name(name)?
+            .set_environment(env)?
+            .into_function()?)
     });
 
     modules.insert("instrument", instrument_loader);