Linking out formatting code

Most of the code remaining in a linked hello world is related to formatting, i.e. ToString() overrides.
DateTime.ToString () alone keeps alive about 100k of IL.

Linking these out automatically is very hard. The BCL is full of calls of Object.ToString () on a receiver
whose type is hard to determine at linking time, like elements of object[].
- For reference types, if there are no instances of the type, the overrides are removed already.
- There are valuetypes with significant formatting code like DateTime. Removing their overrides would
require proving that they have no boxed instances, which is very hard because of generics, i.e. for
every box T, it needs to be proven that T is not a given valuetype.

Probably the best solution right now would be to add some kind of user option, i.e:
- remove these ToString () methods (via the substitution mechanism)
or
- keep these ToString () methods.