* Add metadata to Razor
This PR introduces standard metadata to Razor. This change begins to
formalize the contract between generated code produced by Razor and
runtimes that want to load and interact with this code.
This is a step on making MVC a 'plugin' to Razor rather than the only
possible implementation. Since we're doing MSBuild work - this is the
right time to designate the current interaction between Razor and MVC as
'legacy' and move forward.
Additionally, we need the source thumbprinting to make re-compilation of
Razor a thing.
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Also I noticed that our source document doesn't expose the hash
algorithm by name. We really should have this, so I added it and
hardened the code that deals with checksums in c# generation.
