Static type-checking of required fields.

[judah]

Currently, required fields are defaulted to the "zero" value for that type. We should instead provide smart construction that checks at compile time whether all the required fields have been set properly.

Note that this is moot for proto3, which got rid of the concept of required fields altogether.

One possible, nebulously-described approach: for every datatype Foo, also define a Foo'Builder which is parametrized by the type of each required field (and which may be () if it's not set). This Foo'Builder can be an instance of Default (instead ofFoo), and we can provide lenses to build up its individual fields, as well as a class to "freeze" Foo'Builder into Foo once all its fields have been set.