Added support for type placeholders (explicit requested type
inference in a type with `_` ). This enables partial type inference.
auto merge of #12764 : Kimundi/rust/partial_typehint, r=nikomatsakis

# Summary

This patch introduces the `_` token into the type grammar, with the meaning "infer this type".
With this change, the following two lines become equivalent:
```
let x = foo();
let x: _ = foo();
```
But due to its composability, it enables partial type hints like this:
```
let x: Bar<_> = baz();
```

Using it on the item level is explicitly forbidden, as the Rust language does not enable global type inference by design.

This implements the feature requested in https://github.com/mozilla/rust/issues/9508.

# Things requiring clarification

- The change to enable it is very small, but I have only limited understanding of the related code, so the approach here might be wrong.
  - In particular, while this patch works, it does so in a way not originally intended according to the code comments.
- This probably needs more tests, or rather feedback for which tests are still missing.
- I'm unsure how this interacts with lifetime parameters, and whether it is correct in regard to them.
- Partial type hints on the right side of `as` like `&foo as *_` work in both a normal function contexts and in constexprs like `static foo: *int = &'static 123 as *_`. The question is whether this should be allowed in general.

# Todo for this PR

- The manual and tutorial still needs updating.

# Bugs I'm unsure how to fix

- Requesting inference for the top level of the right hand side of a `as` fails to infer correctly, even if all possible hints are given:

  ```
.../type_hole_1.rs:35:18: 35:22 error: the type of this value must be known in this context
.../type_hole_1.rs:35     let a: int = 1u32 as _;
                                           ^~~~
```