Improve external mutability

[SupunS]

Issue to be solved

A previous version of cadence (secure cadence release) restricted the potential foot-gun of mutating container-typed
let fields/variables via the
Cadence mutability restrictions FLIP.

However, there are still ways to mutate such fields by:

• Directly mutating a nested composite typed field.
• Mutating a field by calling a mutating function on the field.
• Mutating the field via a reference.

Suggested Solution

Explore potential improvements that can be done at the language level to eliminate/minimize the above-mentioned foot guns.

[j1010001]

our recommendation updated onflow/flips#59 - bring up in LDM, decide if we need breakout of if it is ok to close.

[SupunS]

We have three FLIps (onflow/flips#89, onflow/flips#86, onflow/flips#94) proposed for this, and three separate issues (#2534, #2535, #2536) tracking the implementation.

Therefore closing this issue.