codegen should use Expr rather than printing directly to strings

[ExpandingMan]

There are several reasons for this:

• It moves the responsibility of string formatting to the Base Julia code generator. (e.g. breaking printing bugs such as was fixed for empty structs in a few small fixes for codegen #221 could not occur unless they are in Julia itself)
• It can make it easier to use existing metaprogramming and formatting tools within ProtoBuf.jl.
• It can make some fun shenanigans a little easier, such as evaling protobuf code directly into a module. You could do things like dynamically load new protobufs at run-time more easily, though I'm not necessarily claiming that something like this is ever a good idea.

This would require codegen to be completely rewritten, so it's not a very simple change, but it's good to keep in mind in case major changes are ever made to codegen.