Merge curry-base

Fixes #70 (closed) and #88 (closed)

This branch...

• merges curry-base
• splits the frontend into a library and an executable target
• adds the Makefile and the Stack configuration that was previously duplicated between pakcs and kics2

Having this new module structure provides several advantages:

• Combining curry-base and curry-frontend in a single repository simplifies the build process, since compiler repositories can now integrate the frontend using a single submodule
• Cleanly separating executable, library and test targets makes dependency management much easier since the test target no longer has to reenumerate all source modules
• Having a separate library target makes it easy to embed the frontend into third-party applications, e.g. language servers