Scale by read

[Naum-Tomov]

We may want to scale a permission by read, but we may also want to not support it, because there exists an alternative for predicates, e.g. just using Perm or Value.

[bobismijnnaam]

Related: viperproject/silver#534, viperproject/carbon#376, #763

[pieter-bos]

Perhaps good to know in this context: we essentially normalize Perm(_, read) to Value(_) so that read itself does not occur as an expression -- rather the problem then is to define semantics for the result of scaling Perm(_, p) and Value(_) by read.