[emval] Transfer ownership of handles to JS when moveable.

[mrolig5267319]

This is a proof-of-concept that we can reduce the C++ -> JS function call overhead for val temporaries by signaling the decref as part of each of the internal emval functions defined in JS.

This takes advantage of how we currently only use even handles to encode the transfer as an odd handle and centrallying handling this in Emval.toValue.

Without this change a snippet likeval::global()["a"].as<int>() will end up with a pattern like:

handle10 = global
handle12 = "a"
handle14 = 12
decref(14)
decref(12)
decref(10)

With this change we end up reusing handles of the arguments in the results.
handle10 = global
decref(10)
handle10 = "a"
decref(10)
handle10 = 12
decref(10)

The downside is there will be a tedious amount of code expansion to have both lvalue and rvalue reference versions of each method.

@brendandahl @RReverser

[RReverser]

I think @brendandahl fixed some issue related to inconsistent ownership in val calls recently, need to be careful here to make sure we don't regress.