feat: Node 8 support

[malept]

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Summarize your changes:

BREAKING CHANGE: drops support for Node < 8.

Initial Node 8 support. It doesn't really mean that much (since we use TypeScript) other than we can upgrade a bunch of dependencies that also require Node 8.

Closes #790.
Closes #803.
Closes #821.
Closes #822.
Closes #825.