refactor(users/glittershark): Rename to grfn

Rename my //users directory and all places that refer to glittershark to
grfn, including nix references and documentation.

This may require some extra attention inside of gerrit's database after
it lands to allow me to actually push things.

Change-Id: I4728b7ec2c60024392c1c1fa6e0d4a59b3e266fa
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2933
Tested-by: BuildkiteCI
Reviewed-by: tazjin <mail@tazj.in>
Reviewed-by: lukegb <lukegb@tvl.fyi>
Reviewed-by: glittershark <grfn@gws.fyi>

users/grfn/achilles/src/common/env.rs

@@ -0,0 +1,59 @@
+use std::borrow::Borrow;
+use std::collections::HashMap;
+use std::hash::Hash;
+use std::mem;
+
+/// A lexical environment
+#[derive(Debug, PartialEq, Eq)]
+pub struct Env<K: Eq + Hash, V>(Vec<HashMap<K, V>>);
+
+impl<K, V> Default for Env<K, V>
+where
+    K: Eq + Hash,
+{
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+impl<K, V> Env<K, V>
+where
+    K: Eq + Hash,
+{
+    pub fn new() -> Self {
+        Self(vec![Default::default()])
+    }
+
+    pub fn push(&mut self) {
+        self.0.push(Default::default());
+    }
+
+    pub fn pop(&mut self) {
+        self.0.pop();
+    }
+
+    pub fn save(&mut self) -> Self {
+        mem::take(self)
+    }
+
+    pub fn restore(&mut self, saved: Self) {
+        *self = saved;
+    }
+
+    pub fn set(&mut self, k: K, v: V) {
+        self.0.last_mut().unwrap().insert(k, v);
+    }
+
+    pub fn resolve<'a, Q>(&'a self, k: &Q) -> Option<&'a V>
+    where
+        K: Borrow<Q>,
+        Q: Hash + Eq + ?Sized,
+    {
+        for ctx in self.0.iter().rev() {
+            if let Some(res) = ctx.get(k) {
+                return Some(res);
+            }
+        }
+        None
+    }
+}