Add Struct.pre_initialize

[HertzDevil]

See #13481 (comment).

Note that there is no ReferenceStorage(T).pre_initialize, because ReferenceStorage < Value. Simply use T.pre_initialize in that case.

[HertzDevil]

Regarding #13481 (comment), this construction never considers abstract structs; Bar.pre_initialize(Pointer(Foo).new(...)) will not write any type ID, and that Foo will become invalid. It is probably a good idea to disallow abstract structs altogether...?