generates string hashes for arbitrary R objects as follows.
This is the default hash function used by hashsets and hashmaps
objects.
default_hash_fn(key)an arbitrary R object.
a character vector of length one. Hash digest of key.
If key is an atomic vector (as tested by
is.atomic(key)) of length one, default_hash_fn(key) simply
coerces the input to character. For more complex inputs, the function calls
digest(key) from the digest package.