📜 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/eval/value.rs

@@ -1,5 +1,10 @@
 use std::fmt::Display;
 
+/// Values in the interpreter.
+///
+/// Yes, this is yet another definition of a structure called `Value`, which
+/// are almost entirely identical. However, it's nice to have them separated
+/// by type so that we don't mix them up.
 #[derive(Clone, Debug, PartialEq)]
 pub enum Value {
     I64(i64),