napi_remove_wrap()

[gabrielschulhof]

We need to be able to remove a wrap previously applied with napi_wrap().

Observations:

For this to work we need to store the Reference in the External, not the void* because the Reference may be self-destroying or it may not be. Either way, we need to have the Reference for when somebody calls napi_remove_wrap().

We need to document that, if the reference was explicitly requested by calling napi_wrap() with a non-NULL result out-parameter then the reference will become invalid after a call to napi_remove_wrap().

Additionally the implementations of napi_remove_wrap() needs to convert a self-destroying Reference into a non-self-destroying one before destroying it and foremost of all we need to remove the weak reference so the void* explicitly detached from the object prototype does not get double-freed.

[gabrielschulhof]

Closing in favour of #266