Fix warning cppcoreguidelines-pro-type-member-init.

The fmt library happily accepts std::string without
any conversions.

Including
 - usage of static_cast instead of c-style cast,
 - single variable declaration per line,
 - removing const from return types
 - omit unnecessary string to char* conversions,

The used clang-tidy checks are:
google-readability-casting,
readability-const-return-type,
readability-container-size-empty,
readability-delete-null-pointer,
readability-deleted-default,
readability-else-after-return,
readability-function-size,
readability-identifier-naming,
readability-inconsistent-declaration-parameter-name,
readability-isolate-declaration,
readability-misleading-indentation,
readability-misplaced-array-index,
readability-named-parameter,
readability-redundant-*,
readability-simplify-*,
readability-static-accessed-through-instance,
readability-static-definition-in-anonymous-namespace,
readability-string-compare,
readability-uniqueptr-delete-release

Most of the current .clang-tidy contained checks:
google-explicit-constructor
google-readability-braces-around-statements
modernize-loop-convert
modernize-use-default-member-init
modernize-use-emplace
modernize-use-using
readability-braces-around-statements
readability-else-after-return
readability-named-parameter
*readability-namespace-comments
*readability-inconsistent-*
readability-redundant-member-init
readability-simplify-boolean-expr
readability-string-compare
*explicit-conversions*