Avoid allocating intermediate field builder list. (#245)

During encoding, we used to create a list of builders for
the fields of a protobuf message and then make a call to
mconcat on the message builder type.  But mconcat is
recursive and therefore cannot be inlined as such, and
GHC seems reluctant to perform beta reduction on known
constructors such as (:) across module boundaries.

Therefore this commit switches from "mconcat [x, y, z]"
to "mappend (mappend x y) z" (for example).  These
appends are left associativity because the builder
writes in reverse, starting with the rightmost field.

src/Proto3/Suite/DotProto/Generate.hs

@@ -928,7 +928,13 @@ messageInstD stringType ctxt parentIdent msgIdent messageParts = do
                                     [HsPWildCard, HsPRec (unqual_ msgName) punnedFieldsP]
                                     (HsUnGuardedRhs encodeMessageE) []
 
-         encodeMessageE = apply mconcatE [HsList encodedFields]
+         encodeMessageE = case encodedFields of
+           [] -> memptyE
+           (field : fields) -> foldl op (paren field) fields
+             where op fs f = apply (apply mappendE [fs]) [paren f]
+             -- NOTE: We use a left fold because this way the leftmost field
+             -- is the most nested and the rightmost field--the one to be written
+             -- first by the right-to-left builder--is the one that is least nested.
 
          punnedFieldsP = map (fp . coerce . recordFieldName) qualifiedFields
            where fp nm = HsPFieldPat (unqual_ nm) (HsPVar (HsIdent nm))

src/Proto3/Suite/DotProto/Generate/Syntax.hs

@@ -107,10 +107,9 @@ defaultSrcLoc = SrcLoc "<generated>" 0 0
 
 dotProtoFieldC, primC, repeatedC, nestedRepeatedC, namedC, mapC,
   fieldNumberC, singleC, dotsC, pathC, qualifiedC, anonymousC, dotProtoOptionC,
-  identifierC, stringLitC, intLitC, floatLitC, boolLitC, trueC, falseC,
-  nothingC, justC, forceEmitC, mconcatE, encodeMessageFieldE,
-  fromStringE, decodeMessageFieldE, pureE, returnE, memptyE, msumE, atE, oneofE,
-  fmapE :: HsExp
+  identifierC, stringLitC, intLitC, floatLitC, boolLitC, trueC, falseC, nothingC,
+  justC, forceEmitC,  encodeMessageFieldE, fromStringE, decodeMessageFieldE,
+  pureE, returnE, mappendE, memptyE, msumE, atE, oneofE, fmapE :: HsExp
 
 dotProtoFieldC       = HsVar (protobufASTName "DotProtoField")
 primC                = HsVar (protobufASTName "Prim")
@@ -140,10 +139,10 @@ trueC                = HsVar (haskellName "True")
 falseC               = HsVar (haskellName "False")
 nothingC             = HsVar (haskellName "Nothing")
 justC                = HsVar (haskellName "Just")
-mconcatE             = HsVar (haskellName "mconcat")
 fromStringE          = HsVar (haskellName "fromString")
 pureE                = HsVar (haskellName "pure")
 returnE              = HsVar (haskellName "return")
+mappendE             = HsVar (haskellName "mappend")
 memptyE              = HsVar (haskellName "mempty")
 msumE                = HsVar (haskellName "msum")
 fmapE                = HsVar (haskellName "fmap")