Merge #7335 #7691

7335: added region folding r=matklad a=LucianoBestia

Regions of code that you'd like to be folded can be wrapped with  `// #region` and `// #endregion` line comments.
This is called "Region Folding". It is originally available for many languages in VSCode. But Rust-analyzer has its own folding function and this is missing.
With this Pull Request I am suggesting a simple solution. 
The regions are a special kind of comments, so I added a bit of code in the comment folding function.
The regex to match are: `^\s*//\s*#?region\b` and `^\s*//\s*#?endregion\b`.
The number of space characters is not important. There is an optional # character. The line can end with a name of the region.
Example:
```rust
// 1. some normal comment
// region: test
// 2. some normal comment
calling_function(x,y);
// endregion: test
```
I added a test for this new functionality in `folding_ranges.rs`.
Please, take a look and comment. 
I found that these exact regexes are already present in the file `language-configuration.json`, but I don't find a way to read this configuration. So my regex is hardcoded in the code.

7691: Suggest name in extract variable r=matklad a=cpud36

Generate better default name in extract variable assist as was mentioned in issue #1587

# Currently supported
(in order of declining precedence)
1. Expr is argument to a function; use corresponding parameter name
2. Expr is result of a function or method call; use this function/method's name
3. Use expr type name (if possible)
4. Fallback to `var_name` otherwise

# Showcase

![generate_derive_variable_name_from_method](https://user-images.githubusercontent.com/4218373/108013304-72105400-701c-11eb-9f13-eec52e74d0cc.gif)
![generate_derive_variable_name_from_param](https://user-images.githubusercontent.com/4218373/108013305-72a8ea80-701c-11eb-957e-2214f7f005de.gif)

# Questions

* Should we more aggressively strip known types? E.g. we already strip `&T -> T`; should we strip `Option<T> -> T`, `Result<T, E> -> T`, and others?
* Integers and floats use `var_name` by default. Should we introduce a name, like `i`, `f` etc?
* Can we return a list and suggest a name when renaming(like IntelliJ does)?
* Should we add counters to remove duplicate variables? E.g. `type`, `type1`, type2`, etc.


Co-authored-by: Luciano Bestia <[email protected]>
Co-authored-by: Luciano <[email protected]>
Co-authored-by: Vladyslav Katasonov <[email protected]>