Avoid building as root with `sudo make install`

When running `sudo make install`, we only want to run the actual install
as root, the building of the documentation and the distribution folder
should happen as the non-root user.

Related to #13728.
Add help diagnostic messages

This adds ‘help’ diagnostic messages to rustc. This is used for anything that
provides help to the user, particularly the `--explain` messages that were
previously integrated into the relevant error message.
Changed `extern crate foo as bar;` error message

Closes #17709
Fix target-feature codegen option syntax in rustc man page.

The man page stated that the list of features was space-separated when
it's actually comma-separated.
adding iterator benches to tree-based maps for comparison purposes
flesh out BTree docs
doc fixups
librustdoc/html: recognize slices not to nest A tags.

1. A slice of parametrized type, say
   BorrowedRef { ... Vector(Generic(T)) }, is rendered as
   "<a href='primitive.slice.html'>&amp;[T]</a>"
2. A slice of other types, say
   BorrowedRef { ... Vector(int) }, is rendered as
   "<a href='primitive.slice.html'>&amp;[</a>
    <a href='primitive.int.html'>int</a>
    <a href='primitive.slice.html'>]</a>"
3. Other cases, say BorrowedRef { ... int }, are
   rendered as same as before:
   "&<a href='primitive.int.html'>int</a>"

Relevant W3C specs:
- http://www.w3.org/TR/html401/struct/links.html#h-12.2.2
  12.2.2 Nested links are illegal
- http://www.w3.org/TR/html5/text-level-semantics.html#the-a-element
  states A tag must not enclose any "interactive contents"
  which include A tags themselves.
readme: Link 'Using' wiki page instead of 'Getting started'.

The Windows-specific instruction under 'Quick Start' linked the wiki article on getting started developing Rust itself, but the quick start is just about obtaining a working Rust installation. The actual wiki page with Windows-specific instructions was difficult to find.
Fix several issues with the struct and enum representability-checking logic.
In sieve example, end iteration sooner

The Sieve algorithm only requires checking all elements up to and including the square root of the maximum prime you're looking for. After that, the remaining elements are guaranteed to be prime.
rustc: Reformat check_const with modern style

Remove a bunch of two-space tabs
rustc: Add `const` globals to the language

This change is an implementation of [RFC 69][rfc] which adds a third kind of
global to the language, `const`. This global is most similar to what the old
`static` was, and if you're unsure about what to use then you should use a
`const`.

The semantics of these three kinds of globals are:

* A `const` does not represent a memory location, but only a value. Constants
  are translated as rvalues, which means that their values are directly inlined
  at usage location (similar to a #define in C/C++). Constant values are, well,
  constant, and can not be modified. Any "modification" is actually a
  modification to a local value on the stack rather than the actual constant
  itself.

  Almost all values are allowed inside constants, whether they have interior
  mutability or not. There are a few minor restrictions listed in the RFC, but
  they should in general not come up too often.

* A `static` now always represents a memory location (unconditionally). Any
  references to the same `static` are actually a reference to the same memory
  location. Only values whose types ascribe to `Sync` are allowed in a `static`.
  This restriction is in place because many threads may access a `static`
  concurrently. Lifting this restriction (and allowing unsafe access) is a
  future extension not implemented at this time.

* A `static mut` continues to always represent a memory location. All references
  to a `static mut` continue to be `unsafe`.

This is a large breaking change, and many programs will need to be updated
accordingly. A summary of the breaking changes is:

* Statics may no longer be used in patterns. Statics now always represent a
  memory location, which can sometimes be modified. To fix code, repurpose the
  matched-on-`static` to a `const`.

      static FOO: uint = 4;
      match n {
          FOO => { /* ... */ }
          _ => { /* ... */ }
      }

  change this code to:

      const FOO: uint = 4;
      match n {
          FOO => { /* ... */ }
          _ => { /* ... */ }
      }

* Statics may no longer refer to other statics by value. Due to statics being
  able to change at runtime, allowing them to reference one another could
  possibly lead to confusing semantics. If you are in this situation, use a
  constant initializer instead. Note, however, that statics may reference other
  statics by address, however.

* Statics may no longer be used in constant expressions, such as array lengths.
  This is due to the same restrictions as listed above. Use a `const` instead.

[breaking-change]

[rfc]: https://github.com/rust-lang/rfcs/pull/246
core: Convert statics to constants
sync: Convert statics to constants
unicode: Make statics legal

The tables in libunicode are far too large to want to be inlined into any other
program, so these tables are all going to remain `static`. For them to be legal,
they cannot reference one another by value, but instead use references now.

This commit also modifies the src/etc/unicode.py script to generate the right
tables.
collections: Convert statics to constants
libc: Convert all statics to constant

This crate is largely just one giant header file, so there's no need for any of
these values to actually have a memory location, they're all just basically a
regular #define.
rand: Convert statics to constants

This leaves the ziggurat tables as `pub static` as they're likely too large to
want to go into the metadata anyway.
std: Convert statics to constants

This commit repurposes most statics as constants in the standard library itself,
with the exception of TLS keys which precisely have their own memory location as
an implementation detail.

This commit also rewrites the bitflags syntax to use `const` instead of
`static`. All invocations will need to replace the word `static` with `const`
when declaring flags.

Due to the modification of the `bitflags!` syntax, this is a:

[breaking-change]