Separate macro replacement from macro parsing

- Return Vec of replaced tokens
- Add Peekable trait to allow using any `std::iter::Peekable` iterator
- Split up file handling and preprocessing
- Make `impl Debug for InternedStr` actually useful

I was staring at a lot of debugged identifiers while working on this;
I figured they should at least be useful.

- Only perform replacement _after_ processing `defined`
- Document edge cases
- Export `replace`
- Add doc-comments about architecture

src/intern.rs

@@ -4,9 +4,15 @@ use std::sync::RwLock;
 use lasso::{Rodeo, Spur};
 use lazy_static::lazy_static;
 
-#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
+#[derive(Copy, Clone, PartialEq, Eq, Hash)]
 pub struct InternedStr(pub Spur);
 
+impl fmt::Debug for InternedStr {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "{}", self)
+    }
+}
+
 lazy_static! {
     pub static ref STRINGS: RwLock<Rodeo<Spur>> = RwLock::new(Rodeo::default());
     static ref EMPTY_STRING: InternedStr = InternedStr::get_or_intern("");