Merge branch 'develop-copilot-prettyprinter-UpdateField'. Close #526.

**Description**

Copilot currently supports modifying values from streams of structs, but
not pretty-printing them using `copilot-prettyprinter`.

**Type**

- Bug: exception produced when executing valid specification.

**Additional context**

- Issue #520 introduced support for modifying structs.

**Requester**

- Ryan Scott (Galois)

**Method to check presence of bug**

Running the following specification that pretty-prints a spec with a
struct update:

```haskell
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE NoImplicitPrelude #-}
module Main (main) where

import Data.Foldable (for_)
import Data.Functor (void)
import Data.Word (Word32)

import qualified Copilot.PrettyPrint as PP
import Language.Copilot

data S = S
  { unS :: Field "unS" Word32
  }

instance Struct S where
  typeName _ = "s"
  toValues s = [Value typeOf (unS s)]

instance Typed S where
  typeOf = Struct (S (Field 0))

spec :: Spec
spec = do
  let externS :: Stream S
      externS = extern "extern_s" Nothing

      example :: Stream Word32
      example = (externS ## unS =: 42) # unS

  trigger "example" (example == example) [arg externS, arg example]

main :: IO ()
main = do
  spec' <- reify spec
  putStrLn $ PP.prettyPrint spec'
```

produces an error message:

```
$ runghc UpdateFieldPP.hs
UpdateFieldPP.hs: src/Copilot/PrettyPrint.hs:(79,12)-(102,31): Non-exhaustive patterns in case
```

when it should instead execute correctly and pretty-print the spec.

The following Dockerfile, when used with the above file in the context,
installs copilot and runs a spec that uses copilot-prettyprinter to
pretty print an expression with stream with a struct update in it,
followed by "Success":

```
--- Dockerfile
FROM ubuntu:focal

ENV DEBIAN_FRONTEND=noninteractive
RUN apt-get update

RUN apt-get install --yes libz-dev
RUN apt-get install --yes git

RUN apt-get install --yes wget
RUN mkdir -p $HOME/.ghcup/bin
RUN wget https://downloads.haskell.org/~ghcup/0.1.19.2/x86_64-linux-ghcup-0.1.19.2 -O $HOME/.ghcup/bin/ghcup

RUN chmod a+x $HOME/.ghcup/bin/ghcup
ENV PATH=$PATH:/root/.ghcup/bin/
ENV PATH=$PATH:/root/.cabal/bin/
RUN apt-get install --yes curl
RUN apt-get install --yes gcc g++ make libgmp3-dev
RUN apt-get install --yes pkg-config

SHELL ["/bin/bash", "-c"]

RUN ghcup install ghc 9.4
RUN ghcup install cabal 3.2
RUN ghcup set ghc 9.4.8
RUN cabal update

ADD UpdateFieldPP.hs /tmp/UpdateFieldPP.hs

CMD git clone $REPO \
    && cd $NAME \
    && git checkout $COMMIT \
    && cabal v1-sandbox init \
    && cabal v1-install alex happy \
    && cabal v1-install copilot**/ \
    && cabal v1-exec -- runhaskell /tmp/UpdateFieldPP.hs \
    && echo Success
```

Command (substitute variables based on new path after merge):
```
$ docker run -e "REPO=https://github.com/Copilot-Language/copilot" -e "NAME=copilot" -e "COMMIT=<HASH>" -it copilot-verify-526
```

**Expected result**

Running the dockerfile above prints an expression using struct updates
and the message success, indicating that struct updates are supported by
the pretty printer.

**Solution implemented**

Introduce a case for pretty-printing struct updates.

**Further notes**

None.

Author: ivanperez-keera

copilot-prettyprinter/CHANGELOG

@@ -1,3 +1,6 @@
+2024-08-30
+        * Add support for pretty-printing struct update expressions. (#526)
+
 2024-07-07
         * Version bump (3.20). (#522)
 

copilot-prettyprinter/src/Copilot/PrettyPrint.hs

@@ -100,6 +100,9 @@ ppOp2 op = case op of
   BwShiftL _ _ -> ppInfix "<<"
   BwShiftR _ _ -> ppInfix ">>"
   Index    _   -> ppInfix ".!!"
+  UpdateField (Struct _) _ f -> \ doc1 doc2 ->
+    parens $ doc1 <+> text "##" <+> text (accessorName f) <+> text "=:" <+> doc2
+  UpdateField _ _ _ -> impossible "ppOp2" "Copilot.PrettyPrint"
 
 -- | Pretty-print a ternary operation.
 ppOp3 :: Op3 a b c d -> Doc -> Doc -> Doc -> Doc