Avoid extending Base.isopen

The `isopen` function defined here is very different and so it seems
best we define a new function rather than extend `Base.isopen`.
Doing this means we need to export `Intervals.isopen` to make this
change non-breaking however doing this results in:

```julia
WARNING: both Intervals and Base export "isopen"; uses of it in module Main must be qualified
ERROR: UndefVarError: isopen not defined
```

It then seems best to not export `Intervals.isopen` and also then not
export `Intervals.isclosed` to keep things consistent.

Author: omus

src/Intervals.jl

@@ -32,7 +32,6 @@ export AbstractInterval,
        inclusivity,
        first,
        last,
-       isclosed,
        anchor,
        span,
        merge,

src/inclusivity.jl

@@ -38,7 +38,7 @@ Base.first(x::Inclusivity) = x.first
 Base.last(x::Inclusivity) = x.last
 
 isclosed(x::Inclusivity) = first(x) && last(x)
-Base.isopen(x::Inclusivity) = !(first(x) || last(x))
+isopen(x::Inclusivity) = !(first(x) || last(x))
 
 Base.isless(a::Inclusivity, b::Inclusivity) = isless(convert(Int, a), convert(Int, b))
 

src/interval.jl

@@ -114,7 +114,7 @@ Base.last(interval::Interval) = interval.last
 span(interval::Interval) = interval.last - interval.first
 inclusivity(interval::AbstractInterval) = interval.inclusivity
 isclosed(interval::AbstractInterval) = isclosed(inclusivity(interval))
-Base.isopen(interval::AbstractInterval) = isopen(inclusivity(interval))
+isopen(interval::AbstractInterval) = isopen(inclusivity(interval))
 
 ##### CONVERSION #####