Monomorphic word-n-vectors and arrays

[melsman]

This PR aims at providing support for monomorphic WordNVector and WordNArray structures for more values of N than is currently supported (Word8Vector and. Word8Array). For straightforward FFI-support, the scalars inside the vectors and arrays should be represented untagged, also when GC is enabled.

• Rename the file wordtables.sml to polytable.sml and the WordTable functor in the file to PolyTable (applied in Array.sml and Vector.sml).
• Create a generic WordNTable functor that takes sufficient arguments to implement supersets of the MONO_ARRAY and MONO_VECTOR signatures.
• Create a generic WordNTableSlice functor that takes sufficient arguments to implement supersets of the MONO_ARRAY_SLICE and MONO_VECTOR_SLICE signatures.
• Implement backend support for bytetable_sub_wordN and bytetable_update_wordN for N=16, N=31, N=32ub, N=32b, N=63, N=64ub, N=64b.
• Instantiate the functors with arguments appropriate for Word31, Word32, Word64, and Word.
• Implement Word63 structure and instantiate the functors for this word size.
• Implement Word16 structure and instantiate the functors for this word size.
• Instantiate functors on booleans to get BoolVector and friends.
• Vector tests.
• Vector slice tests.
• Array tests.
• Array slice tests.
• New structures RealVectorSlice and RealArraySlice by instantiating functors.
• Many more structures matching the MONO_ARRAY2 signature.

We will deal with int-n-vectors and arrays in a separate PR.