🤷 The initial version of the compiler, both static and JIT.

This implements a full compiler, with both static compilation and JIT
support, for the world's simplest and silliest programming language. You
can do math, and print variables. That's it. On the bright side, it
implements every part of the compiler, from the lexer and parser;
through analysis and simplification; and into a reasonable code
generator. This should be a good jumping off point for adding more
advanced features.

Tests, including proptests, are included to help avoid regressions.

src/ir/strings.rs

@@ -0,0 +1,36 @@
+use super::ast::{Expression, Program, Statement};
+use internment::ArcIntern;
+use std::collections::HashSet;
+
+impl Program {
+    pub fn strings(&self) -> HashSet<ArcIntern<String>> {
+        let mut result = HashSet::new();
+
+        for stmt in self.statements.iter() {
+            stmt.register_strings(&mut result);
+        }
+
+        result
+    }
+}
+
+impl Statement {
+    fn register_strings(&self, string_set: &mut HashSet<ArcIntern<String>>) {
+        match self {
+            Statement::Binding(_, name, expr) => {
+                string_set.insert(name.clone());
+                expr.register_strings(string_set);
+            }
+
+            Statement::Print(_, name) => {
+                string_set.insert(name.clone());
+            }
+        }
+    }
+}
+
+impl Expression {
+    fn register_strings(&self, _string_set: &mut HashSet<ArcIntern<String>>) {
+        // nothing has a string in here, at the moment
+    }
+}