This PR implements the `URLLoader` class, allowing AVM2 scripts
to load data from a URL. This requires several other related
classes (`URLLoaderDataFormat`, `URLRequest`, `IOError`) to be
implemented as well.
Currently implemented:
* Fetching from URLs using the 'navigator' backend
* The `text` and `binary` data formats (which store data
in a `String` or `ByteArray` respectively)
* The `open`, `complete`, and `ioError` events
* The `bytesLoaded`, `bytesTotal`, and `data` properties
Not yet implemented:
* The HTTP and security events
* All of the properties of `IOError`
* The properties on `URLRequest` (besides `url`)
* The "variables" data format
This should be enough to get some basic uses of `URLLoader` working
(e.g. simple GET requests to a particular website).
Note that in Flash's `playerglobal`, the `URLLoader` class is just
a think wrapper around the more general `URLStream`. However,
implementing `URLStream` will require changes to `Navigator``
to support notifications when data arrives in the stream. When
that happens, we should be able to re-use a large amount of the
code in this PR.
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