(PUP-864) Add deprecation warning for mutation of array/hash

This adds a deprecation warning to operations that mutates array
and hash - i.e. $a = [1,2,4] $a[3] = 5, and equivalent for hash.

The warning message has a URL that leads to PUP-864.
Existing tests that test that mutation is possible now also checks
that a warning is issued.
Merge pull request #2160 from hlindberg/pup-864_deprecate-mutating-array-hash-ops

(PUP-864) Add deprecation warning for mutation of array/hash