Make copy / reference semantics more explicit

Implicit copy operations from storage to memory, from calldata to memory, from memory to external function arguments, etc is not possible anymore except for value types.

To make an ordinary value copyable, use the `copyof` operator: `x` cannot be copied, but `copyof x` can be copied. Using `copyof` twice is invalid. Storing a copyable type somewhere erases the copyable property again, i.e. `uint[] memory x = copyof y; c.f(x);` is invalid.

Use the issue's copyof examples as the behavioral specification; determine where copyability and implicit-copy checks are implemented. Done means the listed storage, calldata, memory, and external-argument cases follow the stated rules, including rejection of copyof copyof and loss of copyability after storage.

Written by the indexing model from the issue text.