.ji files should start with "magic" identifier bytes + version

[stevengj]

Currently, the .ji files have no identifying header.

• It is standard practice on Unix for binary files to start with a few bytes (usually an ASCII string) (a "magic number") that identifies the file type (e.g. for the file command). e.g. .ji files could start with "juliaimage".
• It is also good practice to include a version identifier for the file format. e.g. we could have a 4-byte sequence of <major><minor>, and if we detect that the major version is different from what we want then we reject the .ji file as being invalid, whereas changes to <minor> could be assumed backward compatible. (Or don't bother with major/minor and just have a single version number that is required to match.)
• For informational purposes, it might be nice to include the Julia version number that created the file.

[stevengj]

The version number might be redundant, because the .ji file stores the UUID of the Core module (actually just a timestamp) and refuses to load if that doesn't match, so .ji files are effectively specific to a particular Julia build. But it wouldn't hurt to add one.