Remove some useless BUG annotations in the godoc comments.

Fixes #12.
Add a parser benchmark.
Add a field to ast.SyntaxError which contains the stack trace.

And add the -v flag to ast/print which prints the trace on error.
Fixes #19.
Don't tee dot output to out.dot in print.

This was for debugging the dot output. It's debugged.
Use literal token text for keyword and operator token names.

Fixes #20.
Use correct parse error locations in the face of inserted semicolons.

Fixes #21.