Add `instanceClone` parameter to `lang/clone`

Defines the behaviour of cloning instances of custom types
Define `lang/clone` default instance copying behavior
Fix typos for `lang/clone`
Clarify `lang/clone` docs

Specify that it will copy the object reference by default only if the type is
non-native.
Merge pull request #2 from conradz/clone-instances

Add `instanceClone` parameter to `lang/clone`