Prepare and return a Statement, return null on error

Property definitions

sqlite3 $ Sqlite3DB :: prepare
	# Prepare and return a `Statement`, return `null` on error
	fun prepare(sql: Text): nullable Statement
	do
		var native_stmt = native_connection.prepare(sql.to_cstring)
		if native_stmt.address_is_null then return null

		var stmt = new Statement(native_stmt)
		open_statements.add stmt
		return stmt
	end
lib/sqlite3/sqlite3.nit:57,2--66,4