Add support for dynamically sized things in extern splices

[ollef]

Currently, we can't safely splice in dynamically sized things in extern C blocks ((C| $test |)) where the size of test is only known at runtime. We should add support for this, possibly by making the ReturnDirection machinery aware of extern blocks.

We might need some new syntax for splices that need to return by out param. Maybe something like $(out = stuff), where out is a C identifier.

We should also add static checks and errors for unsafe splice uses (e.g. not using the $(out = stuff) syntax when stuff is passed by out param).