Add getindex methods for NamedTuple

[kescobo]

See #38834

This adds getindex() methods for NamedTuples to return multiple values as a new NamedTuple, eg:

julia> nt = (a=1, b=2, c=3)
(a = 1, b = 2, c = 3)

julia> nt[(:a, :b)]
(a = 1, b = 2)

julia> nt[[:a, :b]]
(a = 1, b = 2)

julia> nt[[:a,]]
(a = 1,)

To be analogous with getindex for Tuples and Vectors. When Jeff commented on the issue, he only gave explicit approval to the idea of using a tuple of Symbols to index, not the AbstractVector, but it's easy enough to remove the latter if unwanted. If the AbstractVector method is approved, in the tests, I wasn't sure if testing all of the methods in 1 test would be a good idea or not (see comments) - I assumed keeping them separate would be better.

I also added a bit to the docstring, but I wasn't sure if I should also add something to base/docs/basedocs.jl somewhere around here

Open questions / todos

• OK to keep getindex(t::NamedTuple, idxs::AbstractVector{Symbol}) ?
• If yes, should tests for different methods be kept separate?
  • either way, remove comments in file
• Should I add something to basedocs.jl?
• Should I add a compat notice to docstring like the one here? If yes, I assume it would be for 1.7 at this point?

If there's anything else I've missed, please let me know!

[kescobo]

More stupid typos 🤦 - got it fixed locally but will wait to push so as not to trigger extra CI time

[kescobo]

Bump! Was wondering if maybe failing CI was keeping people from reviewing so I pushed the typo fix.

[quinnj]

Sorry if this has been discussed elsewhere, but why the @inline? Seems to not match the other methods around and should be simple enough inline by itself?

[kescobo]

That's how Jeff wrote it in the attached issue (#38834) - TBH I still don't totally understand what @inline does 🤷 . Happy to leave or remove

[kescobo]

Happy New Year! Bump :-P

[kescobo]

Alright, removed comments from tests, and also rebased onto current master branch. Open questions:

1. should I addto basedocs,jl? I think not - looking again, other methods there are for constructors
2. Should I add compat notice? I think it would be good, but want to double check that it should be for 1.7

[quinnj]

Yeah, add a compat notice

[simeonschaub]

Probably also worth adding an entry in NEWS.md