Add rewriter debugging tool.
Add proper error reporting.
Merge pull request #14 from whitequark/master

Make tuple and ptuple never produce broken ASTs.  "Unary" tuples are inlined, and empty tuples are identified with unit.
Merge pull request #15 from whitequark/rewriter

Add rewriter debugging tool
Make ppx_tools compatible with both 4.02 and trunk.
Use opam 1.2 features.
Bump META version to 0.99.3.
Restore compatibility with 4.02.0.