Problem:    Failing check for dictionary type for const any.
Solution:   Check for any type properly. (closes #11310)

Problem:    Error for modifying a const is not detected at compile time.
Solution:   Add TTFLAG_CONST and check for it in add() and extend().

Problem:    Closure gets wrong value in for loop with two loop variables.
Solution:   Correctly compute the number of loop variables to clear.

Problem:    A closure in a nested loop in a :def function does not work.
Solution:   Use an array of loopvars, one per loop level.

Problem:    Closure doesn't work properly in nested loop.
Solution:   Save variables up to the outer loop.

Problem:    In a :def function all closures in a loop get the same variables.
Solution:   Make a copy of loop variables used in a closure.

Problem:    Not using deferred delete in tests.
Solution:   Use deferred delete more often.

Problem:    Loop variable can't be found.
Solution:   Adjust block_id of the loop variable each round.

Problem:    Vim9: block in for loop doesn't behave like a code block.
Solution:   Use a new block ID for each loop at the script level.

Problem:    Trying to declare g:variable gives confusing error.
Solution:   Give a better error message. (closes #11108)