Replace std.stream dependency in Emitter/Dumper

[kiith-sa]

Similarly to parsing, emitting should write to memory instead of std.stream. Once done, deprecate the std.stream dependent APIs.

[Herringway]

as of dmd 2.069, using std.stream causes the compiler to emit deprecation warnings stating that removal is planned for october 2016

[tmccombs]

I don't think this is an issue anymore. The only reference I can find to std.stream is in the comments.

[wilzbach]

I think this has been fixed as part of 0.5 -> closing