https://github.com/composewell/streamly/blob/master/docs/Build.md

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.

Streamly requires at least cabal 2.2.

Cabal sandboxes were introduced in cabal-install 1.18.

We use Builders, which were introduced in 0.10.0.0.

We don't expect "Path Abs" everywhere anymore. The functions
have been made to be more generic. A user can still pass
absolute paths, so we don't lose any safety. However, some
function implementations may be more tricky.