Duplicated `is` is redundant.

* Add CP.52 and CP.53 guidelines

* Address some feedback on CP.52

* Fix PR break caused by code line longer than 100 characters in CP.53

* Fix spelling error and one line of trailing whitespace

* Tweaks to CP.53 based on feedback

* Clarify "concrete type" and make type category descriptions consistent (closes #1780)

* markdown fixup
Co-authored-by: NJonathan Wakely <cxx@kayari.org>

* Elaborated that we can't generally add comparison operators to C structs
Co-authored-by: NSergey Zubkov <cubbi@cubbi.com>
Co-authored-by: NJonathan Wakely <cxx@kayari.org>

* Changed lettered list to a numbered (isocpp##1800)

* Added blank line for CI (isocpp##1800)

Rewrite C.164 enforcement to flag non-explicit conversion operators only.

This reverts commit de20b33d.

Also fixed "arguments" -> "parameters"

* Remove 2 redundant else, according to rule F.56

* Remove continue from example code

Update after pull request comment.

- fix for D being abstract but trying to create an object of type D in make_unique<D>()

* Minor comment fix

* Minor language fix

* Minor language fix

* Minor clarification

* Minor language fix

* Minor language fix

* Remove whitespace for clarity

* Make whitespace consistent (again, it wasn't the "bad" here, right?)

* Minor language fix

* Change weird formatting for emphasis

* Fix (what I believe is) a slightly distracting typo

* Minor language fix

* Suggest to highlight even security vulnerabilities as a possible outcome of concurrency errors

* Minor language fix

* Suggest to add new section for a seemingly unrelated note

* Minor language fix

* Minor language fix (not sure what this intended to say in the first place?)

* Minor language fix

* Add minor remark

* Minor language improvement

* Minor language fix

* Point out a bad example

* Minor fixes

* Minor language fix

* Add missing period (the lack of which doesn't look bad here, but does a bit in the rendered html)

* Minor language fix

* Minor language fix

* Minor typo fix (but is this what was meant?)

* Minor language improvement (?)

* Minor language fix

* Minor language fix (?)

* Add missing closing quotation mark

* Minor language fix

* Minor language fix

* Remove extra dot (or was it supposed to be three dots instead?)

* Minor language fix

* Minor language fix

* Minor language fix

* Minor language fix

* Minor language improvement

* Minor formatting improvements

* Minor improvements

* Minor language fix

* More fixing

* Add missing parentheses (did I get it right?)

* Minor language fix (?)

* Minor language fix

* Minor language fix

* Fix language (but is this what was meant in the first place?)

* Update heading to match content (in particular, the example)

* Remove superfluous whitespace

* Update also the "bad" example to use int*

Co-authored-by: NBill Rose <neptoess@gmail.com>

Would have been nice to put this after C.9 but item numbers are dense
around here.

* Add '<=>' to comparison operators related rules (C.87, C.167)

C++20's three-way comparison operator should respect the same rules as other
comparison operators.

* Fix a minor typo
Co-authored-by: NJonathan Wakely <github@kayari.org>
Co-authored-by: NJonathan Wakely <github@kayari.org>