fix confusion between unsigned <-> U32

as suggested in #1441.

generally U32 and unsigned are the same thing,
except when they are not ...

case : 32-bit compilation for MIPS (uint32_t == unsigned long)

A vast majority of transformation consists in transforming U32 into unsigned.
In rare cases, it's the other way around (typically for internal code, such as seeds).

Among a few issues this patches solves :
- some parameters were declared with type `unsigned` in *.h,
  but with type `U32` in their implementation *.c .
- some parameters have type unsigned*,
  but the caller user a pointer to U32 instead.

These fixes are useful.

However, the bulk of changes is about %u formating,
which requires unsigned type,
but generally receives U32 values instead,
often just for brevity (U32 is shorter than unsigned).
These changes are generally minor, or even annoying.

As a consequence, the amount of code changed is larger than I would expect for such a patch.

Testing is also a pain :
it requires manually modifying `mem.h`,
in order to lie about `U32`
and force it to be an `unsigned long` typically.
On a 64-bit system, this will break the equivalence unsigned == U32.
Unfortunately, it will also break a few static_assert(), controlling structure sizes.
So it also requires modifying `debug.h` to make `static_assert()` a noop.
And then reverting these changes.

So it's inconvenient, and as a consequence,
this property is currently not checked during CI tests.
Therefore, these problems can emerge again in the future.

I wonder if it is worth ensuring proper distinction of U32 != unsigned in CI tests.
It's another restriction for coding, adding more frustration during merge tests,
since most platforms don't need this distinction (hence contributor will not see it),
and while this can matter in theory, the number of platforms impacted seems minimal.

Thoughts ?

tests/fullbench.c

@@ -64,10 +64,10 @@ static const size_t g_sampleSize = 10000000;
 /*_************************************
 *  Benchmark Parameters
 **************************************/
-static U32 g_nbIterations = NBLOOPS;
+static unsigned g_nbIterations = NBLOOPS;
 static double g_compressibility = COMPRESSIBILITY_DEFAULT;
 
-static void BMK_SetNbIterations(U32 nbLoops)
+static void BMK_SetNbIterations(unsigned nbLoops)
 {
     g_nbIterations = nbLoops;
     DISPLAY("- %i iterations -\n", g_nbIterations);
@@ -316,7 +316,7 @@ static size_t local_ZSTD_decompressContinue(const void* src, size_t srcSize,
 /*_*******************************************************
 *  Bench functions
 *********************************************************/
-static size_t benchMem(U32 benchNb,
+static size_t benchMem(unsigned benchNb,
                        const void* src, size_t srcSize,
                        int cLevel, ZSTD_compressionParameters cparams)
 {
@@ -606,7 +606,7 @@ static int benchFiles(U32 benchNb,
                 benchedSize = (size_t)inFileSize;
             if ((U64)benchedSize < inFileSize) {
                 DISPLAY("Not enough memory for '%s' full size; testing %u MB only... \n",
-                        inFileName, (U32)(benchedSize>>20));
+                        inFileName, (unsigned)(benchedSize>>20));
         }   }
 
         /* Alloc */