Guide: patterns.

Fixes #4417.
Guide: iterators
Add examples for some StrSlice methods.
doc: grammar fixes
doc: small tasks guide improvements
auto merge of #16618 : nham/rust/strslice_examples, r=alexcrichton

(By chance, the `contains` example was actually what I was trying to write when I discovered #16589)
auto merge of #16620 : tshepang/rust/tasks, r=steveklabnik
auto merge of #16621 : tshepang/rust/grammar, r=steveklabnik
auto merge of #16257 : steveklabnik/rust/guide_patterns, r=brson

Fixes #4417.
auto merge of #16408 : steveklabnik/rust/guide_iterators, r=brson

An introduction to iterators. I kinda like this, but I kinda don't. Hmmm.
Guide: macros
fixes some syntax highlighting annotations
Guide: unsafe
auto merge of #16280 : cburgdorf/rust/patch-3, r=steveklabnik
auto merge of #16331 : steveklabnik/rust/guide_macros_and_unsafe, r=brson

The last two sections of the guide, and a small conclusion. I suck at conclusions.

I also realized I never covered strings, so I'm going to put that section up before we're actually 'done.'