Stabilize `const_constructor`
# Stabilization proposal
I propose that we stabilize `#![feature(const_constructor)]`.
Tracking issue: https://github.com/rust-lang/rust/issues/61456
Version target: 1.40 (2019-11-05 => beta, 2019-12-19 => stable).
## What is stabilized
### User guide
Tuple struct and tuple variant constructors are now considered to be constant functions. As such a call expression where the callee has a tuple struct or variant constructor "function item" type can be called:
```rust
const fn make_options() {
// These already work because they are special cased:
Some(0);
(Option::Some)(1);
// These also work now:
let f = Option::Some;
f(2);
{Option::Some}(3);
<Option<_>>::Some(5);
}
```
### Motivation
Consistency with other `const fn`. Consistency between syntactic path forms.
This should also ensure that constructors implement `const Fn` traits and can be coerced to `const fn` function pointers, if they are introduced.
## Tests
* [ui/consts/const_constructor/const-construct-call.rs](https://github.com/rust-lang/rust/blob/
0d75ab2293a106eb674ac01860910cfc1580837e/src/test/ui/consts/const_constructor/const-construct-call.rs) - Tests various syntactic forms, use in both `const fn` and `const` items, and constructors in both the current and extern crates.
* [ui/consts/const_constructor/const_constructor_qpath.rs](https://github.com/rust-lang/rust/blob/
1850dfcdabf8258a1f023f26c2c59e96b869dd95/src/test/ui/consts/const_constructor/const_constructor_qpath.rs) - Tests that type qualified paths to enum variants are also considered to be `const fn`.(#64247)
r? @oli-obk
Closes #61456
Closes #64247