Allow immediate64s in the non-value suffix of mixed records. On its face, this may seem concerning, because immediate64s are boxed on 32 bit platforms. But this is nonetheless OK because mixed records are only represented as mixed blocks (with a flat, non-scannable suffix) on 64 bit platforms.

Changes to the code:

• add Imm64 as an allowed element of the flat suffix
• as a driveby, rename the Float constructor of flat_element to Float_boxed per @mshinwell's suggestion. The old name is easy to confuse with Float32 and Float64. (This change touches a similar set of places in code.)

We have to propagate the immediate64-ness of the field up until lambda. This is point that makes the call as to whether the value is boxed. Assumption: once we hit flambda2, we know that immediate64s are immediates — I believe this assumption is already present in the code base, but I'm less familiar with this part of the world, so I want to flag it explicitly to the flambda2 reviewer.

Changes to auto-generated tests:

• I kick the tests to make them cycle through all of the flat element kinds more eagerly
• I also delete a bunch of repetitive/unnecessary generated tests, and reduce the number of constructor arg tests
  There are also normal unit tests.

I suggest that @ccasin reviews (and I'll get someone else to take a look at the flambda2 pieces afterward). This is non-urgent to review.