feat(ustring): ustring hash collision protection

[lgritz]

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The gist is that the ustring::strhash(str) function is modified to strip out the MSB from Strutil::strhash. The rep entry is filed in the ustring table based on this hash. So effectively, the computed hash is 63 bits, not 64.

But rep->hashed field consists of the lower 63 bits being the computed hash, and the MSB indicates whether this is the 2nd (or more) entry in the table that had the same 63 bit hash.

ustring::hash() then is modified as follows: If the MSB is 0, the computed hash is the hash. If the MSB is 1, though, we DON'T use that hash, and instead we use the pointer to the unique characters, but with the MSB set (that's an invalid address by itself). Note that the computed hashes never have MSB set, and the char*+MSB always have MSB set, so therefore ustring::hash() will never have the same value for two different ustrings.

But -- please note! -- that ustring::strhash(str) and ustring(str).hash() will only match (and also be the same value on every execution) if the ustring is the first to receive that hash, which should be approximately always. Probably always, in practice.

But in the very improbable case of a hash collision, one of them (the second to be turned into a ustring) will be using the alternate hash based on the character address, which is both not the same as ustring::strhash(chars), nor is it expected to be the same constant on every program execution.