Added a future for const checking of array-typed fields
Merge pull request #604 from vasslitvinov/add-const-checking-future

Added a future for const checking of array-typed fields.

From .future:

Const checking is not performed for an array field or a record.

An error needs to be reported either when it is a 'var' field of a 'const' record, or when it is a 'const' field of any record.

TODO: check 'const' field of a 'const' record.

TODO: check when it is a class, not a record.
NB accesses to 'var' array fields of a 'const' class-typed variable
are OK, as opposed to records.