conlang: Rename literals; split, compose, and document Rules

- Renamed literal Types to reflect their literal nature
  - This allows for consistent naming across future non-literal Types
- Complicated lexer Rules have been split into composable sub-rules,
  and moved into the Rule struct.
  - This improves modularity, and allows sharing of sub-rules across rules.
- Documented each lexer rule with (at least) a one-line blurb
  describing its function

.gitignore

@@ -1,2 +1,3 @@
 **/Cargo.lock
 target
+*.pest