📜 Add better documentation across the compiler. (#3)

These changes pay particular attention to API endpoints, to try to
ensure that any rustdocs generated are detailed and sensible. A good
next step, eventually, might be to include doctest examples, as well.
For the moment, it's not clear that they would provide a lot of value,
though.

In addition, this does a couple refactors to simplify the code base in
ways that make things clearer or, at least, briefer.

src/ir/strings.rs

@@ -3,6 +3,10 @@ use internment::ArcIntern;
 use std::collections::HashSet;
 
 impl Program {
+    /// Get the complete list of strings used within the program.
+    ///
+    /// For the purposes of this function, strings are the variables used in
+    /// `print` statements.
     pub fn strings(&self) -> HashSet<ArcIntern<String>> {
         let mut result = HashSet::new();