Add a .text_fast section to IWRAM to allow users to more easily speed up hot code

[zesterer]

Code executing from IWRAM is significantly faster than code executed from ROM. Adding a section in the linker and advice in the book about adding #[link_section = ".text_fast"] to hot functions is likely to help a lot of users achieve better performance.

[Lokathor]

We probably want to call it .iwram, because that's the more commonly used name for it, but yeah this is a good idea.

@zesterer you should do the PR when you have the time :3

[Lokathor]

#118 fixed this