[RFC] sha256.c: clint, -Wconversion, stdbool, and other improvements.

This is the smallest file in neovim's source that lacks the `-Wconversion` flag, and needed only minor changes for clint and to use bool. 

It seems that `sha256_self_test()` might have at one point returned a count of the tests that failed, but anymore it reduces to `FAIL` or `OK` and it doesn't look like anyone has touched it in a very long time.

* Add sha256.c to clint-tests.txt from pull #489.
* Enable `-Wconverion` for issue #567.
* Use `bool` instead of `OK`/`FAIL`. (Doesn't seem to be a specific issue for this.)

Update:
* Remove dead code.
* Don't include vim.h for issue #918.
* Use const.
Yes, thanks. I amended that change.