filesystem: improve read/write apis

[rr-]

Instead of a code like this:

int32_t version;
File_Read(&version, sizeof(int32_t), 1, fp);

this PR lets us write code like this:

const int32_t version = File_ReadS32(fp);

---

This also has the additional benefit of not causing problems if the storage size of the target variable is different than the read size:

int32_t foo = 0x12735471;
File_Read(&foo, sizeof(int16_t), 1, fp); // read two bytes that are both 0xFF
// foo now contains a garbage value of 0x1273FFFF.
// Because we only overrode two bytes, parts of the old value remain intact.

Full example using POSIX C

First off, a sample code, test.c:

#include <stdio.h>
#include <stdint.h>

int main(void)
{
    int32_t value = 0x12735471;
    printf("%x\n", value);
    FILE *fp = fopen("test.txt", "rb");
    fread(&value, sizeof(int16_t), 1, fp);
    printf("%x\n", value);
    fclose(fp);
    return 0;
}

Compiling and examining the results:

$ echo -ne "\xff\xff" > test.txt
$ gcc ./test.c -o ./test
$ chmod +x ./test
$ ./test
12735471
1273ffff

Thanks to having the type promotion work through an assignment, the value of foo in the new API will be correctly set to 0xFFFFFFFF (eg -1 as intended).

Full example using POSIX C

Updated test.c:

#include <stdio.h>
#include <stdint.h>

static int16_t read_s16(FILE *fp)
{
    int16_t result;
    fread(&result, sizeof(int16_t), 1, fp);
    return result;
}

int main(void)
{
    int32_t value = 0x12735471;
    printf("%x\n", value);
    FILE *fp = fopen("test.txt", "rb");
    value = read_s16(fp);
    printf("%x\n", value);
    fclose(fp);
    return 0;
}

Examining the results:

$ echo -ne "\xff\xff" > test.txt
$ gcc ./test.c -o ./test
$ chmod +x ./test
$ ./test
12735471
ffffffff

[walkawayy]

I like the change to return the read variable.