Use portable header identifiers in `#include` statements

Reason

The standard does not specify how compilers uniquely locate headers from an identifier in an #include directive, nor does it specify what constitutes uniqueness. For example, whether the implementation considers the identifiers to be case-sensitive, or whether the identifiers are file system paths to a header file, and if so, how a hierarchical file system path is delimited.

To maximize the portability of #include directives across compilers, guidance is to:

• use case-sensitivity for the header identifier, matching how the header is defined by the standard, specification, implementation, or file that provides the header.
• when the header identifier is a hierarchical file path, use forward-slash / to delimit path components as this is the most widely-accepted path-delimiting character.

Example

// good examples
#include <vector>
#include <string>
#include "util/util.h"

// bad examples
#include <VECTOR>        // bad: the standard library defines a header identified as <vector>, not <VECTOR>
#include <String>        // bad: the standard library defines a header identified as <string>, not <String>
#include "Util/Util.H"   // bad: the header file exists on the file system as "util/util.h"
#include "util\util.h"   // bad: may not work if the implementation interprets `\u` as an escape sequence, or where '\' is not a valid path separator

Enforcement

It is only possible to enforce on implementations where header identifiers are case-sensitive and which only support / as a file path delimiter.