[WIP] Interrealm take 2

[jaekwon]

This replaces the previous interrealm spec.

UPDATE: there are three branches in a chain.

See document for interrealm rules. https://github.com/gnolang/gno/blob/heapitems_for_3693/docs/resources/gno-interrealm.md.

interrealm (this PR, readonly protection)
  <-- interrealm_switchrealm (realm switches #4037)
    <-- heapitems_for_3696 (heap items and many fixes #4060)

heapitems_for_3696 is now effectively the main branch, as it contains a lot of fixes and changes to the VM. Please rebase there.

TODO: update gno-interrealm.md with readonly protection rules from this PR.

1. Keep the check on DidUpdate.
   (storage realm of real objects must be the same as the current realm to edit).
   This not only helps prevent global vars (and its subvalues) from being edited,
   but also values referred to by pointer, by slice (esp []byte), and so on.
2. When code takes a reference (pointer, slice) from a real object persisted
   in an external realm, it is considered "readonly".
   This prevents exploits where such references are passed by argument or even
   as field values to trick the VM into modifying something it doesn't want to.
   (e.g. x := extrealm.Notebook{Counter:&extrealm.Global.Foo[0]}.IncrementCounter())
   Wrapping does not occur by code defined in the same realm as the storage-realm
   of the base. "readonly" is unwrapped for receiver method calls, so methods
   can still modify a value.
3. Make realm switches explicit with 'withrealm(fn)(...)' and corresponding
   'switchrealm()'. 'switchrealm()' can only be used in realms. A function
   or method without 'switchrealm()' does not switch the realm, so /p/ code
   can be copied over to /r/ realms without change in behavior. This allows
   utility methods to exist also in realms.
   'switchrealm()' calls can be stacked, just like master currently.
4. Implicitly "borrow-switch" to the receiver's storage realm when calling a method
   unless there is a switchrealm() in the method, in which case switch
   to the realm in which it is declared. borrow-switching does not affect
   std.CurrentRealm() and std.PreviousRealm().
   This allows new values saved by method to reside in the same realm
   as the receiver, prevents storage fragmentation, and prevents exploits
   related to 1,2 above.
5. Later, introduce attach() uverse function which recursively attaches
   all reachable unreal objects to the caller's realm.
   This is for when you want to pass an unreal object to a caller
   which will hold a reference to the unreal object(s) but you want the
   object to reside in your realm, not the callee's.

This PR is for 1,2 above.
For 3,4 see #4037 and #4060

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[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 42 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
gnovm/pkg/gnolang/ownership.go	50.00%	15 Missing and 1 partial ⚠️
gnovm/pkg/gnolang/op_expressions.go	76.31%	7 Missing and 2 partials ⚠️
gnovm/pkg/gnolang/machine.go	86.95%	6 Missing ⚠️
gnovm/pkg/gnolang/types.go	64.70%	6 Missing ⚠️
gnovm/pkg/gnolang/values.go	92.85%	3 Missing and 1 partial ⚠️
gnovm/pkg/gnolang/op_decl.go	50.00%	0 Missing and 1 partial ⚠️

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[jaekwon]

Comment should say, pop a pointer for writing. All calls to this will end up writing to it.

[jaekwon]

this is vestigial; unless interface value, TV is never undefined, it at least has the concrete value set by defaultValue().

[jaekwon]

after this in separate PR will convert all IsDefined() to IsDefined2().

[jaekwon]

What could go wrong?

[jaekwon]

replaced by #4060