Make macros public to allow easily implementing traits outside of diesel

This makes the expression_impls!, queryable_impls!, and primitive_impls!
public, so programs (or other crates) can more easily implement the
Expression, Queriable, and QueryId traits without duplicating the code
from the macros.

These may change before diesel 1.0, so as discussed in diesel-rs#562, they are hidden
from the documentation for now.

Author: aperezdc

diesel/src/types/impls/mod.rs

@@ -9,6 +9,8 @@ macro_rules! not_none {
     }
 }
 
+#[doc(hidden)]
+#[macro_export]
 macro_rules! expression_impls {
     ($($Source:ident -> $Target:ty),+,) => {
         $(
@@ -56,6 +58,8 @@ macro_rules! expression_impls {
     }
 }
 
+#[doc(hidden)]
+#[macro_export]
 macro_rules! queryable_impls {
     ($($Source:ident -> $Target:ty),+,) => {$(
         impl<DB> $crate::types::FromSqlRow<types::$Source, DB> for $Target where
@@ -90,6 +94,8 @@ macro_rules! queryable_impls {
     )+}
 }
 
+#[doc(hidden)]
+#[macro_export]
 macro_rules! primitive_impls {
     ($Source:ident -> ($Target:ty, pg: ($oid:expr, $array_oid:expr), sqlite: ($tpe:ident))) => {
         #[cfg(feature = "sqlite")]