Change regex! macro to expand to a constexpr, allowing to put it in a static
auto merge of #14423 : Kimundi/rust/static_regex, r=alexcrichton

This patch changes the internals of `Regex` and `regex!()` such that

```rust
static RE: Regex = regex!(...);
```

is valid. It doesn't change anything about the actual regex implementation, it just changes the type to something that can be constructed as a const expression.