Add support for CREATE TABLE ... SELECT ... (#141)

* Heavily renormalizes available overloads of `where()`, `orWhere()`, `having()`, and `orHaving()` for `SQLSelectBuilder`, making them fully consistent with themselves and each other (with the exception of not providing the missing untyped `Encodable` overload for `where()` since it wasn't desirable to have it at all to begin with). Separates the `HAVING` predicate into its own secondary predicate builder.
* Switch to using SQLStatement in SQLCreateTable serialization.
* Add missing SQLSecondaryPredicateGroupBuilder counterpart to SQLPredicateGroupBuilder
* Refactor most of SQLSelectBuilder into a SQLSubqueryClauseBuilder protocol. Despite all appearances to the contrary, the public API does not change (aside from gaining a couple of extra methods for consistency in the `where` and `having` method suites).
* Add support for specifying `CREATE TABLE ... SELECT` queries.
* Organize SQLKitTests.swift (and its async counterpart) a little.
* Add async version of SQLKitTriggerTests for good measure
* Also add support for MySQL's DROP TEMPORARY TABLE syntax, because it's there.

Sources/SQLKit/Builders/SQLCreateTableAsSubqueryBuilder.swift

@@ -0,0 +1,14 @@
+/// A builder used to construct a `SELECT` query for use as part of a `CREATE TABLE` query.
+///
+/// - Note: There's really nothing for this builder to do besides provide a concrete storage
+///   for the `select` property of `SQLSubqueryClauseBuilder`. All of the interesting methods
+///   are on the protocol.
+public final class SQLCreateTableAsSubqueryBuilder: SQLSubqueryClauseBuilder {
+    // See `SQLSubqueryClauseBuilder.select`.
+    public var select: SQLSelect
+
+    /// Create a new `SQLCreateTableAsSubqueryBuilder`.
+    internal init() {
+        self.select = .init()
+    }
+}

Sources/SQLKit/Builders/SQLCreateTableBuilder.swift

@@ -109,6 +109,17 @@ public final class SQLCreateTableBuilder: SQLQueryBuilder {
         createTable.ifNotExists = true
         return self
     }
+
+    /// Specify a `SELECT` query to be used to populate the new table.
+    ///
+    /// If called more than once, each subsequent invocation overwrites the query from the one before.
+    @discardableResult
+    public func select(_ closure: (SQLCreateTableAsSubqueryBuilder) -> SQLCreateTableAsSubqueryBuilder) -> Self {
+        let builder = SQLCreateTableAsSubqueryBuilder()
+        _ = closure(builder)
+        createTable.asQuery = builder.select
+        return self
+    }
 }
 
 // MARK: Constraints

Sources/SQLKit/Builders/SQLDropTableBuilder.swift

@@ -7,10 +7,10 @@ public final class SQLDropTableBuilder: SQLQueryBuilder {
     /// `DropTable` query being built.
     public var dropTable: SQLDropTable
 
-    /// See `SQLQueryBuilder`.
+    // See `SQLQueryBuilder.database`.
     public var database: SQLDatabase
 
-    /// See `SQLQueryBuilder`.
+    // See `SQLQueryBuilder.query`.
     public var query: SQLExpression {
         return self.dropTable
     }
@@ -53,6 +53,14 @@ public final class SQLDropTableBuilder: SQLQueryBuilder {
         dropTable.behavior = SQLDropBehavior.restrict
         return self
     }
+
+    /// If the "TEMPORARY" keyword occurs between "DROP" and "TABLE" then only temporary tables are dropped,
+    /// and the drop does not cause an implicit transaction commit.
+    @discardableResult
+    public func temporary() -> Self {
+        dropTable.temporary = true
+        return self
+    }
 }
 
 // MARK: Connection