Create file systems (with a mount target in each default subnet unless asked
not to), add and remove mount targets, create and delete access points, and
attach a file system to a Lambda function through an access point. A function
not yet in a VPC is put in the file system's mount target subnets with the
default security group, as Lambda requires. TheCloud's mounts are real, so two
functions on one access point share files.
Deleting a file system removes its access points and mount targets first, and
is refused while a function still mounts it.