fileio_types.h : avoid dependency on mem.h

[Cyan4973]

fileio_types.h cannot be parsed by itself
because it relies on basic types defined in lib/common/mem.h.
As for #3231, it likely wasn't detected because mem.h was probably included before fileio_types.h in target files.

An "easy" solution would be to just add the missing include,
but each #include dependency must be considered "bad" by default,
and only allowed if it brings some tangible value.

In this case, since these types are only used to declare internal structure variables
which are effectively only flags,
I believe it's not valuable to add a dependency on mem.h for this purpose
while the standard int type can do the same job.

I was expecting some compiler warnings following this change,
but it turns out we don't use -Wconversion by default on zstd source code,
so there is none.

Nevertheless, I enabled -Wconversion locally and proceeded to fix a few conversion warnings in the process.

Sidenote:
Adding -Wconversion to the list of flags used for zstd is something I would be favorable over the long term,
but it cannot be done overnight,
because the nb of places where this warning is triggered is daunting.
Better progressively reduce the nb of triggered -Wconversion warnings before enabling this flag by default.