Embed standard library into compiler

[dalance]

std is refered by many dependencies, and it is used for interoperation between libraries. So it should be imported by default and just once through a whole project.

There are two ways to import it.

• add https://github.com/veryl-lang/std to dependencies list by default
• embed standard library into compiler

I think the second is better. This is because version management is simpler than the first way, and no network connection is required.

In the second way, compiler will expand std into cache directory, and these files will be added to dependencies list by default.