Merge branch 'develop-remove-tag'. Close #304.

**Description**

The type `Copilot.Core.Expr.Tag` was deprecated in Copilot 3.6 because
it was not used and therefore did not belong in the program.

As per our internal policy of waiting 3 versions from deprecation until
a public interface declaration can be removed, this type can now be
removed completely.

**Type**
- Bug: unused code included in the implementation; a type declared does
  not have a complete API.

**Additional context**

- Issue: #260.

**Requester**

- Ivan Perez

**Method to check presence of bug**

The following expression finds uses of `Tag`, ignoring appearances in comments or strings.
```sh
$ grep -nHre '\<Tag\>' --include='*.hs' | grep -ve '^{-#' | grep -ve '--.*Tag' | grep -ve '"[^"]*Tag[^"]*"'
copilot-core/src/Copilot/Core/Expr.hs:15:  , Tag
copilot-core/src/Copilot/Core/Expr.hs:43:type Tag = Int
```

The result obtained are two lines: the definition, and it being
exported. `Tag`s are not required by Copilot's requirements, and
therefore should not be published.

A complete Dockerfile follows:

```
FROM ubuntu:trusty

RUN apt-get update

RUN apt-get install --yes software-properties-common
RUN add-apt-repository ppa:hvr/ghc
RUN apt-get update

RUN apt-get install --yes ghc-8.6.5 cabal-install-2.4
RUN apt-get install --yes libz-dev

ENV PATH=/opt/ghc/8.6.5/bin:/opt/cabal/2.4/bin:$PWD/.cabal-sandbox/bin:$PATH

RUN cabal update
RUN cabal v1-sandbox init
RUN cabal v1-install alex happy
RUN apt-get install --yes git

CMD git clone $REPO && cd $NAME && git checkout $COMMIT && set -x && grep -nHre '\<Tag\>' --include='*.hs' | grep -ve '^{-#' | grep -ve '--.*Tag' | grep -ve '"[^"]*Tag[^"]*"
```

It can be executed with:
```
$ docker run -e "REPO=<REPO_URL>" -e "NAME=<DIR>" -e "COMMIT=<TAG_BRANCH_OR_HASH>" -it copilot-verify-304
```

**Expected result**

The string returned by the script should not be empty (no type
exists).

Running the docker image will show nothing after a message if the type
is not present.
```
HEAD is now at <TAG_BRANCH_OR_HASH>
```

**Solution implemented**

This change implementation removes that type declaration from the module
`Copilot.Core.Expr`, as well as its export from the module's export
list.

**Further notes**

See
0f536ec
for a confirmation of when the type was deprecated.

Author: ivanperez-keera

copilot-core/CHANGELOG

@@ -1,3 +1,6 @@
+2022-04-07
+        * Remove Copilot.Core.Expr.Tag. (#304)
+
 2022-03-07
         * Version bump (3.8). (#298)
         * Replaces uses of the internal Dynamic with base:Data.Dynamic. (#266)

copilot-core/src/Copilot/Core/Expr.hs

@@ -12,7 +12,6 @@ module Copilot.Core.Expr
   , Expr (..)
   , UExpr (..)
   , DropIdx
-  , Tag
   ) where
 
 import Copilot.Core.Operators (Op1, Op2, Op3)
@@ -38,12 +37,6 @@ type DropIdx = Word32
 
 --------------------------------------------------------------------------------
 
--- | A unique tag for external arrays/function calls.
-{-# DEPRECATED Tag "The type Tag is deprecated in Copilot 3.6." #-}
-type Tag = Int
-
---------------------------------------------------------------------------------
-
 -- | Internal representation of Copilot stream expressions.
 --
 -- The Core representation mimics the high-level Copilot stream, but the Core