Guide: develop the exposition of arrays, vectors, and slices.

The array is the fundamental concept; vectors are growable arrays, and
slices are views into either.  Show common array ops up front: length
and iteration.  Mention arrays are immutable by default.  Highlight
definite initialization and bounds-checking as safety features.  Show
that you only need a type suffix on one element of initializers.
Explain that vectors are a value-add library type over arrays, not a
fundamental type; show they have the same "interface." Motivate slices
as efficient views into arrays; explain you can slice vectors, Strings,
&str because they're backed by arrays.
auto merge of #17962 : jkleint/rust/guide-vectors, r=steveklabnik

The array is the fundamental concept; vectors are growable arrays, and
slices are views into either.  Show common array ops up front: length
and iteration.  Mention arrays are immutable by default.  Highlight
definite initialization and bounds-checking as safety features.  Show
that you only need a type suffix on one element of initializers.
Explain that vectors are a value-add library type over arrays, not a
fundamental type; show they have the same "interface." Motivate slices
as efficient views into arrays; explain you can slice vectors, Strings,
&str because they're backed by arrays.  Show off new, easy-to-read
[a..b] slice syntax.