Fix bug in IWrite causing Idris to not see changes.

[CodingCellist]

IWrite somehow persisently updates the file without requiring a write afterwards. This can lead to unexpected behaviour where Idris fails to recognise that a file has updated and checks the old .ttc version instead of the file containing the change, e.g. a case-split. Adding an explicit write at the end of IWrite seems to fix the problem.