The `get_current_context()` and `get_entered_or_microtask_context()`
methods now return `Option<Local<Context>>` to reflect that an isolate
may not have entered any context.
They're also moved from `Isolate` to `struct Entered` because it turns
out that the underlying V8 API calls actually create new local handles,
hence they should only be used inside an active HandleScope.
The `InContext` trait has been removed.
A test exercising `ContextScope` and the `get_*_context()` methods
mentioned above was added.
Closes: #248.
