Update ETL_CONSTEXPR and C++14 desgination from type_def (#862)
* feat: make type_def constexpr
* test: macro constexpr
* test: implicit constexpr
* test: get constexpr
* refactor: remove constexpr from assignment
* test: comparisons constexpr
* fix: cpp11 support for constexpr get method
* fix: c++11 issue with constexpr get reference
* refactor: remove constexpr from the get methods
* refactor: make assignment operators non-constexpr
* refactor: remove constexpr from assignment and get
* refactor: make other methods ETL_CONSTEXPR14
* test: operator preincrement
* test: postincrement
* test: predecrement constexpr
* test: post decrement constexpr
* fix: post decrement test
* test: addition assignment constexpr
* test: subtraction assignment constexpr
* test: multiplication assignment operator
* test: and assignment constexpr
* test: or assignment constexpr
* test: xor assignment constexpr
* tet: left shift assignment constexpr
* test: right shift assignment constexpr
* test: operator modules assignment
* style: revert personal clang-format changes
* refactor: use free functions to test
* refactor: remove constexpr non-constexpr test functions
* style: remove erroneous tab
