Remove the automatic impl of Expression for Column

I'm tired of seeing rustc recommend that I implement `Column` whenever
I'm having issues resolving `SelectableExpression`. It's making things
hard to debug, and I don't want to wait for
rust-lang/rust#28894 to be fixed.

The error behavior is awfully peculiar. I have no idea why this changed
causes Rust to not even try and compile the second version of the same
expression.

src/macros.rs

@@ -78,6 +78,7 @@ macro_rules! table {
             pub mod columns {
                 use super::table;
                 use $crate::{Table, Column, Expression, SelectableExpression};
+                use $crate::expression::NonAggregate;
                 use $crate::query_builder::{QueryBuilder, BuildQueryResult};
                 use $crate::types::*;
 
@@ -111,6 +112,10 @@ macro_rules! table {
                     }
                 }
 
+                impl SelectableExpression<table> for $column_name {}
+
+                impl NonAggregate for $column_name {}
+
                 impl Column for $column_name {
                     type Table = table;
 

src/query_source/mod.rs

@@ -24,12 +24,6 @@ pub trait Column: Expression {
     fn name() -> &'static str;
 }
 
-impl<C: Column> SelectableExpression<C::Table> for C {
-}
-
-impl<C: Column> NonAggregate for C {
-}
-
 pub trait Table: QuerySource + AsQuery + Sized {
     type PrimaryKey: Column<Table=Self> + Expression + NonAggregate;
     type AllColumns: SelectableExpression<Self>;

tests/compile-fail/cannot_pass_aggregate_to_where.rs

@@ -15,6 +15,4 @@ fn main() {
 
     let source = users.filter(count(id).gt(3));
     //~^ ERROR NonAggregate
-    let source = users.filter(count(id).gt(3));
-    //~^ ERROR E0277
 }

tests/compile-fail/filter_cannot_take_comparison_for_columns_from_another_table.rs

@@ -20,10 +20,6 @@ table! {
 fn main() {
     let _ = users::table.filter(posts::id.eq(1));
     //~^ ERROR SelectableExpression
-    let _ = users::table.filter(posts::id.eq(1));
-    //~^ ERROR E0277
     let _ = users::table.filter(users::name.eq(posts::title));
     //~^ ERROR SelectableExpression
-    let _ = users::table.filter(users::name.eq(posts::title));
-    //~^ ERROR E0277
 }

tests/compile-fail/order_requires_column_from_same_table.rs

@@ -17,5 +17,5 @@ table! {
 
 fn main() {
     let source = users::table.order(posts::id);
-    //~^ ERROR type mismatch
+    //~^ ERROR E0277
 }

tests/compile-fail/select_requires_column_from_same_table.rs

@@ -20,7 +20,24 @@ table! {
 fn main() {
     let connection = Connection::establish("").unwrap();
     let select_id = users::table.select(posts::id);
-    //~^ ERROR type mismatch
-    // ERROR expected struct `posts::table`,
-    // ERROR found struct `users::table` [E0271]
+    //~^ ERROR SelectableExpression
+    //~| ERROR E0277
+    //~| ERROR E0277
+    //~| ERROR E0277
+    //~| ERROR E0277
+    //~| ERROR E0277
+    //~| ERROR E0277
+    //~| ERROR E0277
+    //~| ERROR E0277
+    //~| ERROR E0277
+    //~| ERROR E0277
+    //~| ERROR E0277
+    //~| ERROR E0277
+    //~| ERROR E0277
+    //~| ERROR E0277
+    //~| ERROR E0277
+    //~| ERROR E0277
+    //~| ERROR E0277
+    //~| ERROR E0277
+    //~| ERROR E0277
 }

tests/compile-fail/update_requires_column_be_from_same_table.rs

@@ -24,5 +24,5 @@ fn main() {
     let command = update(users).set(posts::title.eq("Hello"));
     //~^ ERROR type mismatch
     let command = update(users).set(name.eq(posts::title));
-    //~^ ERROR type mismatch
+    //~^ ERROR E0277
 }