Add comment about speed in FAQ.

Fixes #11174.
Guide: pointers

Not the pointer guide, but the guide section on pointers.

Adding a section about patterns for after this, as well.
Be more consistent with "Hello, world!"
auto merge of #16119 : steveklabnik/rust/guide_pointers, r=brson

This is the next section of the guide, and it's on pointers. It's not done yet, as I need to write the section on ownership and borrowing, but I figured I'd share the rest now, to get feedback on the rest of it while I take some time to get that right.
auto merge of #16128 : steveklabnik/rust/speed_faq, r=brson

Fixes #11174.

I'm open to revising this text, but I figured it gets across the basics.
auto merge of #16160 : EduardoBautista/rust/use-bang-at-end-of-hello-world, r=alexcrichton

Further into the guide "Hello, world!" is used instead of "Hello, world".