DbString - analyzer and runtime

[mgravell]

DbString is a Dapper type that allows metadata to be conveyed for string types.

However:

• it is allocatey
• it isn't touched by AOT today
• what it achieves is the same as DbValue]

Suggestions:

1. we need to verify that [DbValue] allows the size and type to be conveyed into DbParameter
2. we should emit a warning to use [DbValue] instead
3. we should probably treat DbString as a recognized type, and configure things correctly
4. for this last, I would propose (if any DbString are encountered) writing a helper method into the output (we have some prior art that copies entire files into the output; that would be fine) with a method that takes a DbParameter and DbString (global:: qualified) and does the right things:

• if the entire DbString is null, just set Value to DbNull
• set the size if appropriate
• set the type if appropriate
• set the value using AsValue, or a (object)value.Value ?? DbNull if easier

The reason we can't put this into the library is that we can't ref Dapper directly because of the Dapper/Dapper.StrongName duality - hence we need to implement it in codegen