Add support for stack allocation of injectables

[HampusMat]

Support for stack allocation of Injectables could probably be implemented.

Stack allocation is to be possible if the interface is a concrete type.

This will require

• Container functions like get to be able to return either a heap allocated value (like Box) or a stack allocated value.
• The generated resolve method of Injectables to figure out whether dependencies should be used as heap or stack values
• Very likely even more