[WORK] std.file.update function

[Walter Bright via Digitalmars-d]

One way to implement it is to open the existing file as a memory-mapped file. 
Memory-mapped files only get paged into memory as the memory is referenced. So 
if you did a memcmp(oldfile, newfile, size), it will stop once the first 
difference is found, and the rest of the file is never read.

Also, only the changed pages of the memory-mapped file have to be written. On 
large files, this could be a big savings.