Allow arbitrary version / checksum comparison expression

[Ortham]

donmac on Discord recently asked if it's possible to write a condition like version('plugin1.esp',version(plugin2.esp), !=). It's not, but I can see it being useful.

It might be worth changing the syntax so that version(path, literal, comparator) becomes version(path) comparator literal like is more usually seen in programming languages. version(path) would return the equivalent of an Option<Version>, where Version is a String that's compared as a version according to the metadata syntax rules. The same would be done for product_version().

The same kind of thing could be done for checksum(), which could be made to return a Option<CRC> where CRC is a String which can only be compared for equality or inequality.