## Changes
* This closes the C++ Style Refactor project.
* Resolves #316.
* Resolves #317.
* Resolves #319.
* Closes #357.
* Reformats all C++ code according to [Google C++ Style Guide].
* Adds `.clang-format` file specifying the exceptions to the Google style guide.
* Adds lint.sh script which prints a diff generated by `clang-format` (if any) and the errors generated by `cpplint`, and allows one to apply formatting in place with `./lint.sh -i`.
* Adds Lint step to CI before Build.
* Updates .gitbub/CONTRIBUTING.md with instructions on how to use lint.sh.
* Adds the missing test file to the Xcode project.

## Comments
* Linting in CI is not parallelized and is using the same Docker image because it will eventually include linting of the Wolfram Language code as well as described in #247.
* Bin packing is now disallowed in addition to the existing style exceptions.
* All `cpplint` errors are fixed as well, not just the formatting.
* `#include <chrono>` is not allowed by `cpplint` for Chromium-specific reasons, so added `// NOLINT` there.
* Renamed `SetTest.cpp` -> `Set_test.cpp` because the suffix appears to be hard-coded to `cpplint`.
* Before the formatting, the code was mostly following this style (imperfectly):
```
---
Language: Cpp
BasedOnStyle: Google
ColumnLimit: 120
BinPackArguments: false
BinPackParameters: false
AllowShortFunctionsOnASingleLine: Empty
IndentWidth: 4
NamespaceIndentation: All
AccessModifierOffset: -4
FixNamespaceComments: false
SpaceAfterTemplateKeyword: false
BreakConstructorInitializers: AfterColon
SpacesBeforeTrailingComments: 1
...
```

## Examples
* Break formatting/code style somewhere in the code, and run `./lint.sh`. For example, we can remove `explicit` on line 22 of Match.cpp, and add a spurious space on the line below:
```
> ./lint.sh 
--- libSetReplace/Match.cpp
+++ formatted
@@ -24 +24 @@
-  bool  operator()(const MatchPtr& a, const MatchPtr& b) const {
+  bool operator()(const MatchPtr& a, const MatchPtr& b) const {

libSetReplace/Match.cpp:22:  Single-parameter constructors should be marked explicit.  [runtime/explicit] [5]
Done processing libSetReplace/Match.cpp
Total errors found: 1
```

* We can run `./lint.sh -i` to fix the space automatically:
```
> ./lint.sh -i
--- libSetReplace/Match.cpp
+++ formatted
@@ -24 +24 @@
-  bool  operator()(const MatchPtr& a, const MatchPtr& b) const {
+  bool operator()(const MatchPtr& a, const MatchPtr& b) const {

libSetReplace/Match.cpp:22:  Single-parameter constructors should be marked explicit.  [runtime/explicit] [5]
Done processing libSetReplace/Match.cpp
Total errors found: 1
```

* The output is the same, however, if we run `./lint.sh` again, there will be no diff:
```
> ./lint.sh   
libSetReplace/Match.cpp:22:  Single-parameter constructors should be marked explicit.  [runtime/explicit] [5]
Done processing libSetReplace/Match.cpp
Total errors found: 1
```

* Finally, if we put explicit back in, there will be no output, which means everything is ok:
```
> ./lint.sh
```

## Changes
* Adds [markdown linter](https://github.com/DavidAnson/markdownlint)
* Enforces 120 characters line width
* Related to #247
* Closes #582

## Examples
Some common errors:
![image](https://user-images.githubusercontent.com/26678747/102560353-08b5a880-40a0-11eb-87be-be2787ccdc6c.png)

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