📜 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.rs

@@ -1,3 +1,17 @@
+//! The middle of the compiler: analysis, simplification, optimization.
+//!
+//! For the moment, this module doesn't do much besides define an intermediate
+//! representation for NGR programs that is a little easier to work with then
+//! the structures we've built from the actual user syntax. For example, in the
+//! IR syntax, function calls are simplified so that all their arguments are
+//! either variables or constants, which can make reasoning about programs
+//! (and implicit temporary variables) quite a bit easier.
+//!
+//! For the foreseeable future, this module will likely remain mostly empty
+//! besides definitions, as we'll likely want to focus on just processing /
+//! validating syntax, and then figuring out how to turn it into Cranelift
+//! and object code. After that point, however, this will be the module to
+//! come to for analysis and optimization work.
 mod ast;
 mod eval;
 mod from_syntax;