UniStr comparison

[bkamins]

1. when hash caching is introduced we could use it to make a first-pass of string comparison try to use it if it is set and do more fancy comparison only if hashes match
2. When comparing the same string, but of different type (eg. ASCIIStr and _UCS2Str - this is in general possible to get it) they compare using === as false. This is probably what we want (though normally identical strings compare as true under Julia 0.7 even if they have a different memory location), but I just want to make sure that this is intended.

[ScottPJones]

I'm not sure I agree with with making === the same as ==, because it eliminates the possibility of shortcuts checking first for === and then for == (which can be slow for different string types).

I don't know where the change was made on master, and what rationale was given.

[bkamins]

Good point. Let us use the rule for === from Base:
compares immutable objects (such as numbers) by contents at the bit level.