Compiler should enforce mutability

[WireWhiz]

If a variable is not marked mut, the compiler should validate that we don't mutate it. Currently we do not. This is important for avoiding runtime errors since immutability is enforced at the page binding level from the runtime.

[WireWhiz]

Update to this, as of https://community.thebrane.org/t/brane-development-log/12/9 we're going to be using mprotect and VirtualProtect like OS functionality to enforce immutability when needed instead of a bindings based solution