The old 'HashTable' type is now the union of the other 3 hash types.
- all operations that used to work on 'HashTable's still work,
but some now have more specific outputs
- `#hash` literals have type `ImmutableHash`
- `immutable?` and `hash-weak?` are filters
- `Mutable-` and `Weak-` hashes have corresponding `Top` types, `HashTableTop` is now a union
- the contact for `(U (Immutable-Hash K1 V1) (Mutable-Hash K2 V2))` is ONE `hash/c`
Minor notes:
- renamed internal identifiers containing 'Hashtable' to all use 'HashTable'
- add Racket guide/reference 'secref' functions
fae58e140d