Add workaround for crystal-lang/crystal#9483

[bcardiff]

After merging #159 I found and issue when using it in the actual drivers.

Building the specs of them leads to a:

BUG: trying to downcast DB::Transaction+ <- DB::TopLevelTransaction (Exception)
  ...

This seems related to crystal-lang/crystal#9483 which is still open.
The workaround is a safe cast before yielding. The cast is safe because of the return type restriction of begin_transaction : Transaction.

Unfortunately there is no way to add a spec for this on crystal-db because DummyDriver is loaded in the specs. DummyDriver defines its own custom transaction type and that, somehow, make the compile error go away also. Drivers do not need to define their own transaction.

By using a foo.cr in the root of crystal-db you can reproduce the issue, as long as DummyDriver is not loaded.

require "./src/db"

DB.open "fake://host" do |db|
  db.transaction do |tx_0|
    tx_0.transaction do |tx_1|
      tx_1.transaction do |tx_2|
      end
    end
    tx_0.rollback
  end
end

Note: there is no FakeDriver defined, and that is fine for this snippet. I was not able to reduce it further.

% crystal foo.cr
BUG: trying to downcast DB::Transaction+ <- DB::TopLevelTransaction (Exception)
  from raise<Exception>:NoReturn
  from raise<String>:NoReturn
  from Crystal::CodeGenVisitor#downcast_distinct<LLVM::Value, Crystal::Type+, Crystal::Type+>:NoReturn
  from Crystal::CodeGenVisitor#downcast<LLVM::Value, Crystal::Type+, Crystal::Type+, Bool>:LLVM::Value
  from Crystal::CodeGenVisitor#visit<Crystal::Var+>:(LLVM::Value | Nil)
  from Crystal::ASTNode+@Crystal::ASTNode#accept<Crystal::CodeGenVisitor>:Nil
  from Crystal::CodeGenVisitor#visit<Crystal::Call>:Bool
  from Crystal::ASTNode+@Crystal::ASTNode#accept<Crystal::CodeGenVisitor>:Nil