feat(api): support more types in where arg to reductions

[jcrist]

All reductions in ibis take an optional columnar where argument - if present, this is used to filter the input before performing the reduction.

Previously we accepted only actual Value inputs here, or deferred expressions. This means that the following would work:

In [1]: import ibis

In [2]: t = ibis.table({"a": "int", "b": "int", "c": "bool"})

In [3]: r1 = t.a.min(where=t.b > 10)  # literal expression

In [4]: r2 = t.a.min(where=_.b > 10)  # deferred expression, resolved against `t`

But not string inputs (would reference columns in the parent table by name), or callables (the lambda form of a deferred expression). Instead, all other inputs were coerced to a boolean (meaning t.a.min(where="bool_col") was surprisingly the same as t.a.min(where=True), but not t.a.min(where=_.bool_col)).

This PR:

• Expands the scope of values that are allowed in where args to column reductions to include direct values (t.bool_col), deferred expressions (_.bool_col), callables (lambda t: t.bool_col), or string column names ("bool_col"). All other inputs are coerced to a literal.
• Applies the same to table reductions (t.count() or t.nunique()). In this case we just make use of the existing bind functionality.
• Adds expression-level tests to check the where arg is being properly handled in all reductions. In several cases it wasn't.

This is a precursor leading to #9170, where we want to add additional args to some columnar reductions that bind to the parent table.

[gforsyth]

Sweet!

I'll let you merge this one, just in case you'd like to rebase and keep the two commits separate.