infer_return_type: simplify definition, use less-private interface

[nsajko]

The Base.promote_op function seems to be less private of an interface than Compiler.return_type. While the single method defined in Base basically just forwards to Compiler.return_type (for the purposes of StableTasks), the semantics are a bit different because a handful of packages actually add other methods to this function. That seems like a good thing, though.

While at it, also avoid defining a new function, instead just assign the callable to a constant binding. Should help code coverage.

[nsajko]

While promote_op is not public:

• it's mentioned in the Manual here

• vtjnash calls it "the documented way to do this" and has made at least one PR to a Julia package to change one to the other: Fix incorrect computation of return_type of itr JuliaArrays/StructArrays.jl#287

[MasonProtter]

I don't like promote_op because it's designed to be overloadable. In many cases that's an advantage, people should be able to tell promote_op things about their function, but I see that as only a negative in the case of StableTasks. I really don't want to get some weird, wrong, or even right answer because someone added a method to promote_op.