Switch from persistent-parser to my own parser module

My parser doesn't support default=, and I think it's safe to assume nobody is
running an instance whose DB schema version is one of those first 5 where
default= exists, so I'm remvoing it from the original 2016 model file. The
unset-default migrations remain, and I checked in `psql` that PostgreSQL
accepts the operation on columns that don't have a default value. If it turns
out to be a problem, I can replace those migrations with no-op ones.

migrations/2016_08_04.model

@@ -19,7 +19,7 @@
 Sharer
     ident   ShrIdent
     name    Text     Maybe
-    created UTCTime        default=now()
+    created UTCTime
 
     UniqueSharer ident
 
@@ -138,7 +138,7 @@ Project
     sharer     SharerId
     name       Text     Maybe
     desc       Text     Maybe
-    nextTicket Int            default=1
+    nextTicket Int
     wiki       RepoId   Maybe
 
     UniqueProject ident sharer
@@ -146,10 +146,10 @@ Project
 Repo
     ident      RpIdent
     sharer     SharerId
-    vcs        VersionControlSystem       default='VCSGit'
+    vcs        VersionControlSystem
     project    ProjectId            Maybe
     desc       Text                 Maybe
-    mainBranch Text                       default='master'
+    mainBranch Text
 
     UniqueRepo ident sharer
 

stack.yaml

@@ -40,10 +40,10 @@ extra-deps:
   - git-0.2.2
   - highlighter2-0.2.5
   - libravatar-0.4.0.2
+  - megaparsec-7.0.5
   - monad-hash-0.1.0.2
   - monadcryptorandom-0.7.2.1
   - patience-0.1.1
-  - persistent-parser-0.1.0.2
   - pwstore-fast-2.4.4
   - time-interval-0.1.1
   - time-units-1.0.0