rustc: Add const globals to the language

[alexcrichton]

This change is an implementation of RFC 69 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]
Closes #17718

[rust-highfive]

Warning

• These commits modify unsafe code. Please review it carefully!

[alexcrichton]

I'll note that this is very much a break-the-world change due to the new restrictions being placed on statics. I don't really know a great way to provide a smooth path forward in all respects, so I didn't put forth any effort in this patch for a deprecation path.

I'll also note that this implements a clarified version of the RFC which may be surprising at first. The PR description/commit messages should have the info though.

I'm always open to suggestions for more tests, I tried to write a test for everything I could think of along the way, but I surely missed something here or there!

[pcwalton]

OK, this looks great overall, just a few nits. r=me with changes, awesome!

[nikomatsakis]

👯