Map keys of type `string16` used `default_hasher_string`, which reads
the length as a byte count; for a `string16` it counts u16 units, so
only the first half of the key was hashed. `cstring16` keys took the
same path and were read as a pointer and a length, which crashes.
Add `default_hasher_string16` and `default_hasher_cstring16` to the
runtime and use them for the generated key hashers, which are also
what the runtime and `intrinsics.type_hasher_proc` use.
bbodi
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