Makefile: Make it easier to re-generate everything

[LukeShu]

When I start looking at contributing to a project, one of the first things I try to answer is "how do I re-build/generate everything that is generated; am I actually able to re-build this?" This PR aims to lower that bar.

• Modify make check_generated to check all generated files, not just README.md
• Merge make gen, make readme, make update_deps, and make assets into a single make generate
• Add make clean to remove non-commited (.gitignored) files that the Makefile generates
• Add make maintainer-clean to remove committed files that the Makefile generates

This mostly aims to document how everything was generated, and the steps necessary to re-generate the same files verbatim; the goal is mostly to figure out how to reproduce the files as they currently are.

To re-generate just an individual file, like README.md, you can just make FILENAME; make README.md will work as expected.

The Makefile now does more work; it downloads the "Dracula" iTerm2 theme for generating demo.svg, it installs the correct version of mockgen for generating log_mock.go, et cetera. All of these "extra" things it puts in a .gitignored directory called tools/. I tried to keep the Makefile readable and understandable, though.

I rolled back the version of github.com/golang/mock from v1.1.1 to v1.0.0, since that's the version used to generate log_mock.go, and mixing versions of mockgen outputs vs mock libraries seemed unwise.

I adjusted the test suite to work with GO111MODULE=on, which the Makefile sets.

[CLAassistant]

All committers have signed the CLA.

[LukeShu]

The "GolangCI" check failure is "expected" and has nothing to do with the changes in this PR.

Commit 692dacb added dupImport, but since that commit isn't yet in a stable release, GolangCI isn't using it yet, so it complains. I guess that CI failure is expected until the next release happens.

[jirfag]

the fantastic work, thank you very much!