Fixing node12 deprecation warnings

[haata]

• NewFromUtf8
• Get
• Set
• ToString

[kentonv]

Hmm, what V8 really wants people to do is to stop calling GetCurrent() all over the place and instead pass down the isolate/context/etc. from the FunctionCallbackInfo. So this feels a little sad to deal with all the deprecations by working around them.

Also sad to be using Check() all over the place instead of propagating the exception, which is what V8 really wants us to do.

But, not sure how much effort it's worth to try to do it right...

[haata]

I might have some time to work on cleaning things up, still learning how V8 is put together.
I've been trying to track down a compiler/linker error on Arch Linux and was hoping to cut down on a bunch of the deprecation noise.

(My end goal is to integrate capnproto into an electron app).