CSE machine: choice of ':' vs ':=' should be done when scanning out declarations and not when running asgn control instruction

[wpinrui]

Currently, all names are initialised with :, and asgn instructions are treated differently for const and let declarations.

When a let declaration is evaluated, the value is bound to the name.
When a const declaration is evaluated, the same happens but there is additionally a = placed to the right of the :.

Thus, in a program where a const declaration never gets evaluated but its block is reached, we have the situation where the environment diagram indicates : instead of :=:

(https://share.sourceacademy.nus.edu.sg/n1012)[https://share.sourceacademy.nus.edu.sg/n1012]