feat: allow dicts in ibis.literal for structs

[cpcloud]

Currently a struct must be an OrderedDict, but since 3.6 there's no reason we can't allow at least dicts any possibly any mapping:

>>> import ibis
>>> ibis.literal({"a": "b", "c": "d"}, type="struct<a: string, c: string>")
TypeError: Value {'a': 'b', 'c': 'd'} cannot be safely coerced to struct<a: string, c: string>

[jjram20]

I would like to take this issue. /take

[cpcloud]

@jjram20 Cool, just write only /take!

[jjram20]

/take

[jjram20]

Hi, did it work now?

[jjram20]

/take

[cpcloud]

The action might be backed up in the CI queue, give it a few minutes.

[saulpw]

We're going to keep ibis.literal() as it is for constructing plain literal expressions (int, float, string, etc), and add new constructors for compound literal expressions, just like the existing ibis.array():

• ibis.struct()
• ibis.map()

These new constructors take a Python dict as the value, and an optional type kwarg like ibis.literal() (this type kwarg is also added to ibis.array() for symmetry). struct() creates a literal value expression in which the field names and value types are fixed, and the value types can be heterogenous within the same struct. map() creates a literal value expression in which the types of the keys and values are fixed and homogenous, but which can have any set of key/value pairs (and any number of them).

literal() will still continue to work as it has (e.g. it needs an OrderedDict specifically to construct a StructValue), but these special constructors should be preferred for non-plain literals in new code.