base/env: Implementation of 'Expanding_parent'
init: Transfer slack memory to children on demand
base: Use distinct error for ref_account mismatch
core: Don't account RAM metadata to RAM quota

The meta data for allocations should be accounted to the session quota
instead of the RAM accont managed by the session.
core: Fix off-by-one in quota check

The check was one byte too rigid. ;-)
base: Add 'Env::finalize_initialization'
Test for 'Parent::resource_request' interface