Provide a function to write the standard ebml header

[catamorphism]

as per FIXME in std::ebml.

[metajack]

Would this be the "EBML Basics" defined here? http://ebml.sourceforge.net/specs/

I don't see anything about "standard header" in the specs here, so I figured I'd ask for clarification.

[emberian]

Visiting for triage. FIXME is still there but I have no idea what the standard header should be either.

[huonw]

Triage: FIXME is still there; is the "standard header" the EBML Basics section of the Element Semantics table of @metajack's link?

[emberian]

See also #9303, which will obviate this (since it will now be our own datatype, it will probably see a revamp and not have and weirdness like this)

[emberian]

http://matroska.org/technical/specs/rfc/index.html, see section 5. This defines what we probably should be doing here.