This fixes various problems with doctest:

* https://github.com/sol/doctest/issues/245
* https://github.com/composewell/streamly/issues/83

Also clean up travis.yml for cabal-3.

This reverts commit 21dd1718c0.

We use Builders, which were introduced in 0.10.0.0.

This allows to specify the behavior on recursive operations,
such that one can collect failures instead of dying on the first
failure.

We don't really need this.

We now create the necessary directories and files
for the tests on-the-fly.

* add isFileName
* add hasParentDir
* add hiddenFile
* add our own openFd version for more control
* small documentation improvements
* add a getDirectoryContents' version that works on Fd
* fix linting warnings
* lift version constraints in benchmark

Also adjust HPath.IO to work with the new API.