remove the chap-NNN- convention from file names

I don't want those showing up in the URLs.

Author: nikomatsakis

src/SUMMARY.md

@@ -1,20 +1,20 @@
 # Summary
 
-- [About this guide](./chap-000-about-this-guide.md)
-- [How to build the compiler and run what you built](./chap-010-how-to-build-and-run.md)
-- [Using the compiler testing framework](./chap-020-running-tests.md)
-- [Walkthrough: a typical contribution](./chap-030-walkthrough.md)
-- [High-level overview of the compiler source](./chap-040-high-level-overview.md)
-- [The parser](./chap-050-the-parser.md)
-- [Macro expansion](./chap-060-macro-expansion.md)
-- [Name resolution](./chap-070-name-resolution.md)
-- [HIR lowering](./chap-080-hir-lowering.md)
-- [Representing types (`ty` module in depth)](./chap-090-ty.md)
-- [Type inference](./chap-100-type-inference.md)
-- [Trait resolution](./chap-110-trait-resolution.md)
-- [Type checking](./chap-120-type-checking.md)
-- [MIR construction](./chap-130-mir-construction.md)
-- [MIR borrowck](./chap-140-mir-borrowck.md)
-- [MIR optimizations](./chap-150-mir-optimizations.md)
-- [trans: generating LLVM IR](./chap-160-trans.md)
+- [About this guide](./about-this-guide.md)
+- [How to build the compiler and run what you built](./how-to-build-and-run.md)
+- [Using the compiler testing framework](./running-tests.md)
+- [Walkthrough: a typical contribution](./walkthrough.md)
+- [High-level overview of the compiler source](./high-level-overview.md)
+- [The parser](./the-parser.md)
+- [Macro expansion](./macro-expansion.md)
+- [Name resolution](./name-resolution.md)
+- [HIR lowering](./hir-lowering.md)
+- [Representing types (`ty` module in depth)](./ty.md)
+- [Type inference](./type-inference.md)
+- [Trait resolution](./trait-resolution.md)
+- [Type checking](./type-checking.md)
+- [MIR construction](./mir-construction.md)
+- [MIR borrowck](./mir-borrowck.md)
+- [MIR optimizations](./mir-optimizations.md)
+- [trans: generating LLVM IR](./trans.md)
 - [Glossary](./glossary.md)