add keyword arguments to IOBuffer's constructors

[bicycle1885]

This adds constructors of IOBuffer taking keyword arguments (e.g. write=true) as open does, which will supersede #24430.

TODO:

• Update docs.
• Deprecate old constructors that accept positional arguments?
• Update constructor calls if we deprecate positional arguments.

[bicycle1885]

I think this is ready to review. Please ignore the last commit, which just removes a junk file I made accidentally.

I'm still wondering whether we should choose IOBuffer(write=true) or IOBuffer(read=true, write=true, truncate=true) by default. I think the former is preferable since it is minimal and consistent to open but it is breaking because it makes IOBuffer() no longer readable by default:

julia> Base.open_flags(write=true)
(read = false, write = true, create = true, truncate = true, append = false)

julia> Base.open_flags(read=true, write=true, truncate=true)
(read = true, write = true, create = true, truncate = true, append = false)

Any comments? @vtjnash @StefanKarpinski

[JeffBezanson]

I don't think this is true --- the keyword arguments default to nothing, which open_flags interprets as read-only. And that's probably a good thing; for example we have

IOBuffer(str::String) = IOBuffer(unsafe_wrap(Vector{UInt8}, str))

which we want to remain read-only.

[bicycle1885]

You are right. I meant "When data is not given, ...". Fixed.

[StefanKarpinski]

The examples don't include the truncate flag. As an aside, I guess I should write the code to figure out a minimal set of keywords to reproduce a particular set of open flags – this output is a bit on the long side.

[bicycle1885]

Thank you. I added some examples in #25919.

[JeffBezanson]

Not important, just wondering: since truncate is also passed to the constructor above, is this redundant?

[bicycle1885]

Yes, it is. Fixed in #25919.

[stevengj]

Passing a StringVector like this seems to be pretty common. It would be good to add a sizehint keyword argument so that you could just do IOBuffer(sizehint=sizeof(s)) here and in similar places.

[bicycle1885]

Okay, I'll take care of it.