break out parts of the region inference chapter into sub-chapters

Author: nikomatsakis

src/SUMMARY.md

@@ -76,6 +76,10 @@
             - [Move paths](./borrow_check/moves_and_initialization/move_paths.md)
         - [MIR type checker](./borrow_check/type_check.md)
         - [Region inference](./borrow_check/region_inference.md)
+            - [Constraint propagation](./borrow_check/region_inference/constraint_propagation.md)
+            - [Placeholders and universes](./borrow_check/region_inference/placeholders_and_universes.md)
+            - [Closure constraints](./borrow_check/region_inference/closure_constraints.md)
+            - [Errror reporting](./borrow_check/region_inference/error_reporting.md)
         - [Two-phase-borrows](./borrow_check/two_phase_borrows.md)
     - [Constant evaluation](./const-eval.md)
         - [miri const evaluator](./miri.md)