Implement in-place huge allocation shrinking.

Trivial example:

    #include <stdlib.h>

    int main(void) {
        void *ptr = malloc(1024 * 1024 * 8);
        if (!ptr) return 1;
        ptr = realloc(ptr, 1024 * 1024 * 4);
        if (!ptr) return 1;
    }

Before:

    mmap(NULL, 8388608, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fcfff000000
    mmap(NULL, 4194304, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fcffec00000
    madvise(0x7fcfff000000, 8388608, MADV_DONTNEED) = 0

After:

    mmap(NULL, 8388608, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f1934800000
    madvise(0x7f1934c00000, 4194304, MADV_DONTNEED) = 0

Closes #134
add back support for mremap via MREMAP_RETAIN