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
Guard opaque predicates with an (-> Any Any) contract. This uses the
contract generation infrastructure to avoid wrapping struct predicates.
Also, relax `any-wrap/c` (the contract used for `Any` in positive
position) to allow opaque structures. This also requires an enumeration
of all the other kinds of values that TR understands, so that they are
not confused with opaque structures.
Joint work with @bennn.
Closes#202.
Closes#203.
Closes#241.
