I made a typo in `str::levdistance` in my str pull request. This fixes it.

Closes #5411.

Closes #4468

r? @catamorphism

This makes the `trim` and `substr` functions return a slice instead of an `~str`, and removes the unnecessary `Trimmable` trait (`StrSlice` already contains the same functionality).

Also moves the `ToStr` implementations for the three str types into the str module in anticipation of further untangling.

In struct section of tutorial, make everything more coherent and
clear by always using "struct Point".
Also, do not prematurely introduce pointers and arrays.
Fixes #5240

Closes #4468

librustc: Forbid destructors from being attached to any structs that might contain non-Owned fields. r=nmatsakis

The old string benchmarks weren't very useful because the strings weren't long enough, so I just threw those out for now. I left out benchmarks of `oldmap` because it's clear that it's 30-40% slower and it doesn't implement the `Map` trait.

This also cleanly divides up `insert`, `search` and `remove`.

Adds an assert_eq! macro that asserts that its two arguments are equal. Error messages can therefore be somewhat more informative than a simple assert, because the error message includes "expected" and "given" values.

Switched over substr and trim functions in str to be non-allocating, temporary renamed them to better track use-sites

* Closes #4603

In struct section of tutorial, make everything more coherent and
clear by always using "struct Point". Also, do not prematurely
introduce pointers and arrays. Fixes #5240
Signed-off-by: NLuca Bruno <lucab@debian.org>

r? @nikomatsakis The typechecker previously passed around a boolean return flag to
indicate whether it saw something with type _|_ (that is, something
it knows at compile-time will definitely diverge) and also had some
manual checks for the `ty_err` pseudo-type that represents a previous
type error. This was because the typing rules implemented by the
typechecker didn't properly propagate _|_ and ty_err. I fixed it.

This also required changing expected error messages in a few tests,
as now we're printing out fewer derived errors -- in fact, at this
point we should print out no derived errors, so report any that
you see (ones that include "[type error]") as bugs.

r? @catamorphism

r? @pcwalton 

A slice now always refers to something that returns an borrowed pointer, views don't exist anymore. If you want to have an explictit copy of a slice, use `to_owned()`

Referencing #4986.