Refactor String header layout reflection

[straight-shoota]

This patch removes the assumption of Strings memory layout in stdlib when allocating an instance by hand.
The exact memory layout is governed by the compiler (or rather the LLVM backend takes care of the specifics). The assumed layout is probably correct most of the time, but it's not guaranteed.
And it's easy to implement in a way that does not assume anything about the memory layout except that the char array is the last field.
This action decouples the stdlib implementation from compiler internals.

It also paves a way for potential future changes to Strings layout, for example as suggested in #12020.

[straight-shoota]

I added a commit that re-uses Object.set_crystal_type_id instead of a custom implementation.
The bug fixed in #13338 does not have any effect (same as in WeakRef.allocate) because the first four bytes are overridden by the bytesize afterwards.