Module Extended.AccessSource
Access defines an access token to inspect and use data within a capsule
'k t represents access to the current capsule. An uncontended 'k t indicates that 'k is the current capsule. A shared 'k t indicates that 'k is the current capsule, but that it may be shared with other threads. A contended 'k t indicates that 'k might be some other capsule, and will thus grant no capabilities.
'k ts cross portability, which means they can be shared with other threads. However, they do not cross contention, which means that 'k ts captured in a portable closure become contended, thus preventing any access to its data
packed is the type of access to some unknown capsule. Unpacking one provides a 'k t together with a fresh existential type brand for 'k.
A boxed version of Access.t for places where you need a type with layout value.
Obtain an Access.t for the current capsule. Since we do not know the brand for the current capsule, we receive a fresh one.