Float32 array load/store primitives

[TheNumbat]

Adds %primitives for loading/storing float32s to/from strings, bytes, bigstrings, and float32_elt bigarrays. Also covers loading/storing unboxed float32s to/from bigstrings.

The string, bytes, and bigstring primitives have the same semantics as those for int32s; the only difference is that they operate on a float32 register directly. This did not require any compiler backend changes.

The bigarray functions are specialized copies of caml_ba_ref / caml_ba_set for float32. They require bigarrays with the float32_elt element kind, but purposefully ignore the bigarray's 'view' type. This allows the API functions to operate on e.g. (float, float32_elt, 'c) t bigarrays.

• This required adding a new bigarray kind in the compiler, Pbigarray_float32_t / Float32_t. This kind not exposed by the standard library, and is only used by the new primitives.
• When generating calls to generic caml_ba_get_$N / caml_ba_set_$N, we now need to check whether the kind is Pbigarray_float32_t. This was necessary because the default runtime functions will convert float32_elts to doubles.
• Only 1, 2, and 3 dimensional float32_elt bigarrays are supported. Generic arrays must still convert through float.

Tests for all new primitives have been added to float32_lib.ml and float32_u_lib.ml.

p.s. fixes an assembly emission problem with CVTSI2SS, @xclerc to review the last commit

[mshinwell]

@TheNumbat please add a full PR description

[TheNumbat]

Yeah, added, but this still isn't ready for review. Something bad is going on with the cfg backends