build: allow skipping of the hash check in the download step

This is useful when trying to bisect an issue in a dependency, and we
want to pull from a specific commit but don't want to have to download
and determine the sha1sum and md5sum of the tarball.
Fix outdated msgpack-rpc doc.

Needed after neovim/python-client#58.
Merge pull request #1554 from jszakmeister/allow-skipping-of-sum

build: allow skipping of the hash check in the download step
Merge pull request #1556 from elmart/fix-msgpack-rpc-docs

Fix outdated msgpack-rpc doc.
Remove remaining crypto references

Closes https://github.com/neovim/docs/pull/26

Also added stubs for 'cryptmethod' and 'key', and placeholders for
explanation regarding removal of crypto functionality.