* Create templated abstract classes KeysView, ValuesView and ItemsView, and implement them on-the-fly when wrapping any specific map type

* We don't want to wrap different ValuesView objects for double values and const double, for example, as both wrappers will be named ValuesView[float]

* Fallback to C++ names if key or values types are not wrapped

* Added a test for .keys(), .values() and .items() returning the same types for similarly-typed maps

* Fixed wrong use of auto in a declarator list: the two descriptions might have different types

* Fixes for clang-tidy issues: explicit single-argument constructor, using the 'override' keyword when overriding functions

* Bugfix for old versions of clang++, which seem to have trouble with the struct being defined inside a module, which was also needlessly ugly anyway

* Bugfix for clang++, which doesn't have some of the names in runtime uness they are specified to be static

* A fix for clang-tidy performance-inefficient-string-concatenation issues - I personally think this looks uglier, but it's probably worth it for clang-tidy to be happy

* Possible fix for clang++ linking issues - make the descriptions static constexpr to make sure they are known before linking

* Correct names for previously-wrapped types as keys/values of maps

* Bugfix - typo in type info names which caused things to segfault

* Apply suggestions from code review

Co-authored-by: Aaron Gokaslan <skylion.aaron@gmail.com>

* Use detail::remove_cvref_t instead of doing remove_cv and remove_reference separately

* Avoid names with double underscore, as they are reserved

* Improved testing for KeysView, ValuesView and ItemsView: check type names + stricter asserts

* Moved description logic to helper function in type_caster_base.h

* style: pre-commit fixes

* Fix a clang-tidy issue: do not use 'else' after 'return'

* Apply suggestion by @Skylion007, with additional trivial simplification.

Co-authored-by: Amir <aimir@local>
Co-authored-by: aimir <aimir@localhost>
Co-authored-by: Aaron Gokaslan <skylion.aaron@gmail.com>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Ralf W. Grosse-Kunstleve <rwgk@google.com>