Stop quoting column default values (#200)

When adding a column to a table (either at creation time or afterwards
with an alter table migration), any default value for the column is
automatically single quoted.

This makes it easier to define migrations, as the user does not have to
remember to add the single quotes in the migration.
For example in this migration:

```json
{
  "name": "28_different_defaults",
  "operations": [
    {
      "create_table": {
        "name": "items",
        "columns": [
          {
            "name": "id",
            "type": "serial",
            "pk": true
          },
          {
            "name": "name",
            "type": "varchar(255)",
            "default": "unnamed"
          }
        ]
      }
    }
  ]
}
```
the default value for the `name` column (`unnamed`) does not need to be
quoted as `pgroll` does that automatically.

However, this automatic quoting causes a problem when assigning a
default value of `now()` to a timestamp column:

```json
{
  "name": "28_different_defaults",
  "operations": [
    {
      "create_table": {
        "name": "items",
        "columns": [
          {
            "name": "id",
            "type": "serial",
            "pk": true
          },
          {
            "name": "created_at",
            "type": "timestamptz",
            "default": "now()"
          }
        ]
      }
    }
  ]
}
```
when run, this will result in a `created_at` column with a default value
of the time at which the migration was run, eg:
```
| created_at | timestamp with time zone |  not null default '2023-11-06 08:37:01.203691+00'::timestamp with time zone | 
```

rather than the desired default:
```
| created_at | timestamp with time zone |  not null default now() |
```

This PR removes the automatic quoting of default values so that `now()`,
`CURRENT_TIMESTAMP` etc can be used correctly as column defaults. This
pushes some complexity onto users who now have to single-quote those
default values that require it.

docs/README.md

@@ -925,6 +925,7 @@ Example **create table** migrations:
 * [19_create_orders_table.json](../examples/19_create_orders_table.json)
 * [20_create_posts_table.json](../examples/20_create_posts_table.json)
 * [25_add_table_with_check_constraint.json](../examples/25_add_table_with_check_constraint.json)
+* [28_different_defaults.json](../examples/28_different_defaults.json)
 
 ### Drop column
 

examples/28_different_defaults.json

@@ -0,0 +1,32 @@
+{
+  "name": "28_different_defaults",
+  "operations": [
+    {
+      "create_table": {
+        "name": "items",
+        "columns": [
+          {
+            "name": "id",
+            "type": "serial",
+            "pk": true
+          },
+          {
+            "name": "name",
+            "type": "varchar(255)",
+            "default": "'unnamed'"
+          },
+          {
+            "name": "price",
+            "type": "decimal(10,2)",
+            "default": "0.00"
+          },
+          {
+            "name": "created_at",
+            "type": "timestamptz",
+            "default": "now()"
+          }
+        ]
+      }
+    }
+  ]
+}

pkg/migrations/op_create_table.go

@@ -127,7 +127,7 @@ func ColumnToSQL(col Column) string {
 		sql += " NOT NULL"
 	}
 	if col.Default != nil {
-		sql += fmt.Sprintf(" DEFAULT %s", pq.QuoteLiteral(*col.Default))
+		sql += fmt.Sprintf(" DEFAULT %s", *col.Default)
 	}
 	if col.References != nil {
 		sql += fmt.Sprintf(" CONSTRAINT %s REFERENCES %s(%s)",