Similar to create() in LibJS, wrap() et al. are on a low enough level to
warrant passing a Realm directly instead of relying on the current realm
from the VM, as a wrapper may need to be allocated while no JS is being
executed.
Just some boilerplate code to get started :^)
This adds both the SubtleCrypto constructor to the window object, as
well as the crypto.subtle instance attribute.
Since we don't support IDL typedefs or unions yet, the responsibility
of verifying the type of the argument is temporarily moved from the
generated Wrapper to the implementation.
