Fix deprecation: Use symbol method for custom inspection

[eritbh]

Setting inspect() on objects in order to modify the behavior of util.insect(obj) is deprecated and prints a warning as of Node 10. The alternative, [util.inspect.custom](), has been available since node 6.

Deprecation ref: nodejs/node#15549

This outdates the typescript defs - I don't know how to document symbol methods so I didn't touch them. If anyone else can, let me know and I'll update this PR.

[Brayzure]

I think this should be bundled in with node 6 changes, whenever we get to them. Additionally, we should probably grant users the same courtesy that node has, and deprecate this before removing it entirely.

[eritbh]

What other Node 6 changes are there? Not familiar with that.

I guess that's a fair point, but I dunno if there's any case where it matters - does anyone use #inspect outside the util.inspect use?

If it does matter, that can be resolved by setting the two properties to the same value to silence the deprecation warning (the symbol version takes precedence): Base.prototype.inspect = Base.prototype[util.inspect.custom] would be backwards-compatible and not emit the warning on 10.x.

[abalabahaha]

Agree with Brayzure, no sense breaking Node 4 compatibility for this single change. Probably would be nicer to do it the other way around - set prototype[util.inspect.custom] to prototype.inspect if the symbol exists.