Change :float to {:f, 64} and add {:f, 32} dtype

[philss]

The idea is to support more dtypes in the way they are represented in Arrow/Polars.

In the near future, we won't need to "normalise" numeric columns anymore, and by consequence, Explorer is going to use less memory if needed.

This is related to #471

[philss]

I feel that we may want to have a "shortcut" version for declaring the dtypes, like in Nx.
We could accept either :float for {:f, 64} or :f32 and :f64.

Also, the inspection of the series and dataframes could be different, and follow Nx as well - just f32 or f64 -, or slightly larger, like float64.

WDYT @josevalim @cigrainger @billylanchantin ?

[josevalim]

I think we should definitely still support :float for backwards compatibility. We can add f32/f64 later, it should be a quick addition. Also, please add tests that to_iovec with f32 works. Other than that, this is great, ship it!

[josevalim]

I think we should definitely still support :float for backwards compatibility.

Suggestion: rename valid_dtype? to normalize_dtype (which returns nil if no dtype) and rename validate_dtype to normalize_dtype, which returns the normalize dtype. Now change all call sites of normalize_dtype and normalize_dtype! to consider its return value (which is the normalized dtype). Then it should be easy to add :float, but I think we can do that in the next PR. So let's just add the test for to_iovec with f32 to this one and the normalize stuff can come in a separate one!

[cigrainger]

Yep I agree -- let's keep the 'simple' names like :float and give them defaults as they are today. Otherwise this is fantastic. Re: inspection, I prefer to follow Nx as closely as possible.

[philss]

@josevalim @cigrainger I made changes applying your suggestions. Please take a look.

The only thing I didn't change was to add the aliases :f32 and :f64 for the float dtypes. I'm planning to do that in another PR tomorrow, with docs as well.

[cigrainger]

Dropped one Q but otherwise this looks fantastic. I'm excited to support more explicit datatypes, esp w/ zero-copy conversion to tensors.

[cigrainger]

I wonder if we shouldn't do something like:

Suggested change

	- Simply typed series: `:binary`, `:boolean`, `:category`, `:date`, `:datetime`, `:duration`, `{:f, 32}`, `{:f, 64}`, `:integer`, `:string`, and `:time`.
	- Simply typed series: `:binary`, `:boolean`, `:category`, `:date`, `:datetime`, `:duration`, `:float` (`{:f, 32}` or `{:f, 64}`), `:integer`, `:string`, and `:time`.

WDYT?

[philss]

Agreed! I added more mentions of it in other places as well: 3605cfc