Shuffle some types

Put types in the modules they belong to instead of including them in
each one from the header.

src/geef_oid.erl

@@ -7,15 +7,18 @@
 
 -include("geef_records.hrl").
 
+-type oid() :: #geef_oid{oid :: binary()}.
+-export_type([oid/0]).
+
 -export([parse/1, hex/1]).
 
 %% @doc Get a hex-encoded string of the hash
--spec hex(geef_oid()) -> binary().
+-spec hex(geef_oid:oid()) -> binary().
 hex(#geef_oid{oid=Oid}) ->
     geef_nif:oid_fmt(Oid).
 
 %% @doc Parse an iolist as a hash
--spec parse(iolist() | binary()) -> geef_oid().
+-spec parse(iolist() | binary()) -> geef_oid:oid().
 parse(Sha) ->
     Oid = geef_nif:oid_parse(Sha),
     #geef_oid{oid=Oid}.