8371297134 Type promotion between integral and floating types

[poodlewars]

To fix the bug reported in 8371297134

• Write an int64 column
• Append a float32 to it
• Read it -> We blow up at read time

I've added logic so that when we merge descriptors we combine:

any integer + float 64 -> float 64
integer up to 16 bits + float 32 -> float 32
integer above 16 bits + float 32 -> float 64

This is because a 16 bit integer can safely fit in a 32 bit float without loss of precision, whereas a 32 bit integer cannot. We could instead always promote up to float 64 which would be more wasteful but simpler.

Separately, for query builder pipelines I added changes so that when combining a float and an integer we always promote up to float64. This misses the cases where it is actually safe to promote to float32, but is simpler and matches Pandas. This is the cause of the test change in test_sort_merge.py.

I reworked has_valid_type_promotion, introducing a new function is_valid_type_promotion_to_target instead. This is because the has_valid_type_promotion signature was dangerous - it returned an optional type that was often interpreted only as a bool (the actual type inside it was ignored). I've replace the call sites that only tested the bool with calls to the new is_valid_type_promotion_to_target that only returns a bool.

[alexowens90]

This is technically a semver change of behaviour

[alexowens90]

Could move to nonreg tests

[poodlewars]

This change is just to help debugging tests