util: rz_endian cleanup and documentation (#3900)

* librz: rename function parameter

The function parameter is renamed from _dest to dest. In the body of
the function, the identifiers _dest and dest are swapped
accordingly. This makes these functions have the same parameter name
as all others of the same kind in the header.

* librz: add swap macros when builtins missing

The rz_swap_ macros are only defined when the compiler builtins are
available. We define them in case they are not for uniformity of
presence in the preprocessor namespace.

* librz: add documentation for rz_endian.h

Every function and macro is documented except for the signed swap
macros.

* librz: return max value in read if parameter is NULL

The read functions in rz_endian.h sometimes returned the MAX value
when the source parameter was NULL and sometimes didn't. For example,
compare rz_read_be16 which doesn't check NULL and rz_read_le16 which
does.

We fix this by inserting checks for NULL in every integer read
function.

We also insert these checks for the floating-point read
functions. They also too sometimes checked for NULL indirectly by
calling the corresponding integer read functions, but the read
functions with the offset parameter did not check for NULL.

Separately, The NULL checks in rz_read_le8 and rz_read_me8 were
removed because they were duplicate; they call rz_read_ble8 which
checks for NULL.

* librz: add documentation in swap functions for NULL source parameter

The case where the source parameter is NULL is documented in all the
read functions in rz_endian.h

A mistake in the documentation of rz_read_at_ble64 is fixed; it
incorrectly mentioned 128-bit instead of 64-bit integers.

* librz: add missing 24-bit macros

The 24-bit macros for minimum and maximum value are added.

* librz: add missing endianness functions

For the readers and writers, certain endianness and bit number
combinations were missing. Some missing big-endian, little-endian,
middle-endian functions as well as helper functions are added.

The documentation of some read blue functions is corrected to mention
"specified" order instead of "big-endian".

* librz: fixing some compiler errors from typos

* librz: remove UT24 macros

We remove the UT24_ macros due to PR review.

* librz: replace UT24_MAX with UT32_MAX

It is better to return the maximum value held by the return type
instead of returning a max 24-bit value.

* test: add more rz_endian tests

All the integral endian functions for read and write are ran through
some basic tests.

* librz: remove unused internal functions

The signed rz_swap functions are removed because they are not used
anywhere in the source code. They are duplicates of the corresponding
unsigned functions. Moreover, the rz_swap_st16 function has an
incorrect implementation when the compiler builtin is missing.

Instead of fixing, adding documentation, and unit tests, which means
we would also need to create an environment with the compiler builtin
check undefined, we simply remove them, because they are not used
anywhere.

* librz: remove unnecessary define macros

These macros were added during the PR and were requested to be removed
again.

* test: fix MSVC complaining on erroneous commas

The string literal #__VA_ARGS__ is equal to "" in a conforming
implementation when there is an empty list of variadic arguments. On
MSVC 2017, it is completely missing, causing an error. These issues
appear to have been fixed in VS2019 v16.5 and later using the
/Zc:preprocessor compiler switch and in Visual Studio 2017 version
15.8 using /experimental:preprocessor.

In any case, we use string concatenating to force an empty string to
appear even if #__VA_ARGS__ is missing.