[#42] Migrate to lts-15.7

Description

[#42] Migrate to LTS-15.7

Problem: it's 2020 already and GHC-8.8 is out, but we still use 8.6
branch.

Solution: use LTS-15.7 with GHC-8.8.3. Support newer `singletons`. Clean
all new warnings.

Related issue(s)

Resolves #42 (closed)

✅ Checklist for your Merge Request

Related changes (conditional)

• Tests (see short guidelines)

  • If I added new functionality, I added tests covering it.
  • If I fixed a bug, I added a regression test to prevent the bug from silently reappearing again.

• Documentation

  • I checked whether I should update the docs and did so if necessary:
    • Root README and other README.md files
    • Haddock
    • docs/
  • I updated changelog files of all affected packages released to Hackage if my changes are externally visible.

Stylistic guide (mandatory)

• My commits comply with the following policy.
• My code complies with the style guide.