Kernel returns copy of object literals, preventing updates from applying

[RomainMuller]

As reported in aws/aws-cdk#774, the jsii kernel returns copies of object literals when coalescing them to a particular named type (typically an interface). This makes updates to interface-typed properties not reflect to the object they were obtained from.

A CDK example is when reading options from an @aws-cdk/cdk.Resource instance, and attempting to update it.