start to implement new models
re-impl the test computation expression
add new assertions using new model
re-impl the trap computation expression
rewrite PersimmonTest using new model
re-impl the parameterize computation expression
add Syntax.fs
remove Persimmon.fs
add Writable.fs

eliminate the Writer from new model.
we just use the TextWriter.
add Formatter.fs
improve the SummaryFormatter.normal
change the format of the ProgressFormatter.dot
add Printer.fs and Reporter.fs
add ErrorFormatter.normal
re-impl Runner using new model (incompleted)

Old Runner is devided into TestCollector and TestRunner.
remove unused module.
fix runtime errors
fix console output

We need a line break at the end of the progress report.
change the type of ContextResult.Children from seq to list

Now, we can execute the Sample.fs.
implement aggregation the count of error tests