Add a hash function

[charlesstaats]

Add a hash function for strings. This hash function is based on std::hash, but with two notes:

• To get a 62-bit hash (which is the largest number of bits in Asymptote such that every possible bitstring can be represented as a positive int), the two most-significant bits are first XOR'd with the two least significant bits.
• Since std::hash is implementation-defined, its values are likely to change from time to time, and may also differ e.g. across operating systems. To prevent users from coming to rely on a consistent but not-promised behavior, the hash is also XOR'd with a 62-bit integer that is randomly chosen each time asy starts up (but remains consistent throughout a single run of asy). A try_consistent option is available for users who want to disable this enforced inconsistency.

The result is a new builtin function <int hash(string s, bool try_consistent=<default>)>.

[user202729]

If we have taken the effort to write such a thing, why not make it cryptographically secure? (might need an extra dependency on OpenSSL though for maybe SHA256?)

[charlesstaats]

I ended up adding a bunch more commits to this branch, so this pull request no longer really makes sense.