rearrange to promote the borrow checker into its own section

Author: nikomatsakis

src/SUMMARY.md

@@ -50,9 +50,9 @@
     - [MIR construction](./mir/construction.md)
     - [MIR visitor and traversal](./mir/visitor.md)
     - [MIR passes: getting the MIR for a function](./mir/passes.md)
-    - [MIR borrowck](./mir/borrowck.md)
-      - [MIR-based region checking (NLL)](./mir/regionck.md)
     - [MIR optimizations](./mir/optimizations.md)
+- [The borrow checker](./borrow_check.md)
+    - [Region inference](./borrow_check/region_inference.md)
 - [Constant evaluation](./const-eval.md)
     - [miri const evaluator](./miri.md)
 - [Parameter Environments](./param_env.md)