roundup / down `Irrational` endpoints

I propose that `a .. b` should round `a` down and `b` up if they are irrational, so that `a in a..b` and `b in a..b` is always true. This would fix the following:
```julia
julia> π in 0..π
false

julia> π in (0..Float64(π, RoundUp))
true
```

Reproduce the Julia examples using `a .. b`, `Irrational` endpoints, `Float64(..., RoundUp)`, and `in`. Then inspect the interval-construction and membership entry points to determine how irrational endpoints are represented and compared. Done means both endpoint membership checks are true without requiring manual rounding.

Written by the indexing model from the issue text.