Define Module as a replacement for HsModule

[toku-sa-n]

Description of the PR

This PR defines Module as a replacement for HsModule.

The purpose is to mitigate the burden caused by GHC's version changes. GHC changes the structure of its AST when it's updated, which almost always requires us to modify our pretty-printing functions, including locating CPP blocks, duplicating existing pretty-printing functions, and fixing many compile errors. Also, when we want to change the pretty-printing style, we need to modify multiple CPP blocks for just a change. By creating HIndent's own AST, we can keep up with the latest GHC with less bothersome because we only need to update our AST while we can change the printing style more easily as we don't need CPP pragmas in Pretty instances of our AST types.

This PR is complete in the sense that it doesn't change any existing behaviors, but is incomplete because we need to replace the other AST types from GHC. Still, I chose to create multiple small PRs for easier reviewing.

Checklist

• Add a changelog if necessary. See https://keepachangelog.com/ for how to write it.
• Add tests in TESTS.md if possible.