v0.6.x back patch: add node v12 support

[kumavis]

Goal:

provide node v12 support to v0.6.x range without breaking backwards compat

Problem

no package with native perf supports node v6 and node v12 (meeting backwards compat + goal)

Proposal:

use scryptsy pure js implementation, and add an npm deprecation notice encouraging (upgrading or hard coding to the previous version)

Cons:

lose native perf speed boost

Pros:

really good compatibility, it just works. also prods people to upgrade to latest

[kumavis]

🦊 ethereumjs-wallet@0.6.x appears in MetaMask's dep graph in a few places, and is blocking our node v12 migration MetaMask/metamask-extension#8245

[kumavis]

@holgerd77 let me know what you think 💡

[holgerd77]

Hmm, I think this is a no-go TBH since it is taking away major functionality components within a bugfix release. Any other options or suggestions?

#108 can' be taken over for Node compatibility reasons?

[holgerd77]

Slept over it and I think I can live with it now. It's some kind of weird mixture of properties for a bugfix release, but might be the most pragmatic solution here after all. So let's do it.

Can you prepare a release PR similar to e.g. here on this, and can you please also mention the upcoming v1.0.0 ("in the next 1-2 weeks") release? Then I will merge and publish on npm.