rustc_typeck: don't expect rvalues to have unsized types.
Rename and namespace `FPCategory`

Rename `FPCategory` to `FpCategory` and `Fp* to `*` in order to adhere to the
naming convention

This is a [breaking-change].

Existing code like this:
```
use std::num::{FPCategory, FPNaN};
```
should be adjusted to this:
```
use std::num::FpCategory as Fp
```

In the following code you can use the constants `Fp::Nan`, `Fp::Normal`, etc.
auto merge of #20083 : eddyb/rust/fix-expectation, r=nikomatsakis

This fixes a few corner cases with expected type propagation, e.g.:
```rust
fn take_int_slice(_: &[int]) {}
take_int_slice(&if 1 < 0 { [ 0, 1 ] } else { [ 0, 1 ] });
```
```rust
<anon>:2:28: 2:36 error: mismatched types: expected `[int]`, found `[int, ..2]`
<anon>:2 take_int_slice(&if 1 < 0 { [ 0, 1 ] } else { [ 0, 1 ] });
                                    ^~~~~~~~
<anon>:2:46: 2:54 error: mismatched types: expected `[int]`, found `[int, ..2]`
<anon>:2 take_int_slice(&if 1 < 0 { [ 0, 1 ] } else { [ 0, 1 ] });
                                                      ^~~~~~~~
```
Right now we unpack the expected `&[int]` and pass down `[int]`, forcing
rvalue expressions to take unsized types, which causes mismatch errors.
Instead, I replaced that expectation with a weaker hint, for the unsized
cases - a hint is still required to infer the integer literals' types, above.

Fixes #20169.
Add structs for ranges to core::ops.
Add lang items for ranges.
Add syntax for ranges
Type checking and trans for ranges
Tests
Review changes
auto merge of #19758 : tbu-/rust/pr_fp_name, r=alexcrichton

This is a [breaking-change].
auto merge of #19858 : nick29581/rust/ranges, r=aturon

Closes #19794

r? @aturon for the first patch
r? @nikomatsakis for the rest