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.
* Change sha256_update to accept a size_t for length.
* Don't include vim.h in sha256 (issue #918).
* Narrow #includes.
* Use const* when not modifying argument (sha256_update and _process).