1. A non-transitory, computer-readable storage medium storing program instructions that when executed on a computing device cause the computing device to perform: loading, at runtime by a runtime module executing within a runtime environment, a class from a runtime linkable, multi-version library, wherein the multi-version library is configured to include multiple versions of classes, wherein all versions of a respective class correspond to an identical class signature for the respective class; wherein said loading comprises: determining whether the multi-version library includes a version of the class associated with a current version of a software development kit for implementing software executable within a respective current version of the runtime environment; and in response to determining that the multi-version library does include the version of the class associated with the current version of the software development kit, loading, from the multi-version library, the version of the class associated with the current version of the software development kit.