cogent: fix #64 -- vim syntax highlighting

[skip ci]

Rename everything COGENT to Cogent(2nd pass).

In the earlier attempt(`a65975f31627e2ea215498510cc231fe805d495f`) to rename
traces of `COGENT` to `Cogent`, I seemed to have missed out on a few instances.
This patch attempts to fix that.
Hopefully, this time around, we get a the regression tests to pass.

Code reorganisation.

* A rejig of the the Cogent compiler's directory structure.
* Support for building Cogent compiler with both `cabal` and `stack`.
(`stack` support is EXPERIMENTAL)
* Use Makefiles for build and install.
* Remove trailing white-spaces.
* Generate `dummy headers` in `tests/include` for testing C code.
* Remove unused `gen_types.sh` script
* Bump version to `2.0.6`

TODO:
* Remove duplicated files in `libgum`