sha256.c: Enable -Wconvert and clint.

* Add sha256.c/h to clint-files.txt.
* Add sha256.c to CONV_SRCS in src/nvim/CMakeLists.txt.
* Use size_t for lengths and const for read-only parameters.
* Use restrict keyword.
* Refine #includes.
sha: define SHA256_BUFFER_SIZE and DIGEST_SIZE.