Verify integrity of downloaded libraries

[wiktorn]

• split fetch_unpack into separate fetch and unpack functions
• provide fetch_unpack using above two for backward compatibility
• add build_simple_check that uses separate fetch/unpack functions and checks SHA256 after fetching. This is provided as new function, as the interface has changed (instead of version it takes variable prefix) so old one is left for backwards compatibility
• change all build_* to use build_simple_check or fetch/unpack and check SHA256 depending on previous approach
• change variables for OPENSSL_* so it's using standard variables (OPENSSL_VERSION and OPENSSL_HASH instead of OPENSSL_DOWNLOAD_URL, OPENSSL_ROOT and OPENSSL_HASH)

Closes: #308

[wiktorn]

And also I've made some changes according to https://www.shellcheck.net/ (at least, where I was already changing something)

[wiktorn]

Should I care about these two failed jobs? It looks to me that it's something broken in Homebrew?