Remove <string> includes in headers to improve build time

[dementive]

The <string> header was in class_db.hpp and method_bind.hpp even though no parts of it are used in either of those files. I also removed the <iostream> header from class_db.hpp because it was also unused. Both of these files ended up getting included in thousands of other cpp files so slowed down build time a lot.

There seem to be only 2 uses of code from <string> in godot-cpp as the code works now:

1. In src/godot.cpp it was used for printf and snprintf. Removing from the class_db header broke this but just including stdio fixed it.
2. It was also used in templates/char_traits.cpp for std::char_traits::length. I left the include in this file because I didn't want to change how it works and it's in a cpp file so it doesn't matter that much.

I did 2 test runs where I ran with ClangBuildAnalyzer and the time command then did scons --clean. These are the timings I got before and after making these changes:

Without these changes

Executed in 108.61 secs
Executed in 107.91 secs

**** Time summary:
Compilation (1025 times):
Parsing (frontend): 1701.6 s
Codegen & opts (backend): 187.1 s

With these changes

Executed in 97.04 secs
Executed in 91.19 secs

**** Time summary:
Compilation (1025 times):
Parsing (frontend): 1393.1 s
Codegen & opts (backend): 181.2 s

[Ivorforce]

Makes sense to me. We do not use std::string with godot-cpp. So it should be good as long as it compiles.

[Calinou]

Code looks good to me.